LTC4126
1
Rev. A
For more information www.analog.comDocument Feedback
TYPICAL APPLICATION
FEATURES DESCRIPTION
7.5mA Wireless Li-Ion Charger with
1.2V Step-Down DC/DC Converter
The LT C
®
4126 is a low-power wireless single-cell Li-Ion
battery charger with an integrated step-down DC/DC regu-
lator. The step-down regulator is a low-noise multi-mode
charge pump which is powered from the battery and pro-
vides a regulated 1.2V at the output. The switching fre-
quency is set to either 50kHz or 75kHz depending on the
mode to keep any switching noise out of the audible range.
The LTC4126 charger is a full-featured constant-current
constant-voltage Li-Ion battery charger with automatic
recharge, automatic termination by safety timer, and bat-
tery temperature monitoring via an NTC pin. Charge cur-
rent is fixed at 7.5mA with a 6-hour termination timer.
Undervoltage protection disconnects the battery from all
loads when the battery voltage is below 3.0V.
The tiny 2mm × 2mm LQFN package and minimal external
component count make the LTC4126 well suited for Li-Ion
battery powered hearing aid applications and other low power
portable devices where a small solution size is required.
Top and Bottom View of the IC with
Complete Application Circuit
APPLICATIONS
n Wireless Li-Ion Battery Charger Plus High
Efficiency Multi-Mode Charge Pump DC/DC
n Wideband Rx Frequency: DC to >10MHz
n Integrated Rectifier with Overvoltage Limit
n Pin Selectable Charge Voltage: 4.2V or 4.35V
n Charge Current: 7.5mA (Fixed)
n Low Battery Disconnect: 3.0V
n NTC Pin for Temperature Qualified Charging
n DC/DC Regulated Output: 1.2V
n DC/DC Output Current: Up to 60mA
n 50kHz/75kHz Switching, No Audible Noise
n Pushbutton and/or Digital on/off Control for DC/DC
n Thermally Enhanced 12-Lead 2mm × 2mm
LQFNPackage
n Hearing Aids
n Low Power Li-Ion Powered Devices
n Wireless Headsets
n IoT Wearables All registered trademarks and trademarks are the property of their respective owners.
V
IN
LTX LRX
8µH
CRX
68nF
2.2µF
1.2V
TRANSMITTER
AIR GAP
ACIN
OUT
VSEL
GND
Li-Ion
4.2V
4126 TA01
LTC4126 BAT
NTC
CHRG
VCC
+
–
+
ACPR
STAT2
DIGITAL I/O
STAT1
EN
TO BAT
LTC4126
2
Rev. A
For more information www.analog.com
PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
Input Supply Voltages
VCC........................................................... –0.3V to 6V
ACIN ..........................................................–10V to 6V
ACIN – VCC Differential ...........................–16V to 0.3V
Input/Output Currents
IACIN ................................................................ 200mA
IOUT ................................................................. –60mA
BAT .............................................................. –0.3V to 6V
PBEN, NTC, EN,
VSEL ...........................–0.3V to [Max (VCC, BAT) + 0.3V]
CHRG ........................................................... –0.3V to 6V
Operating Junction Temperature Range ... –20°C to 85°C
Storage Temperature Range .................. –40°C to 125°C
Maximum Reflow (Package Body)
Temperature ..........................................................260°C
(Notes 1, 2)
LQFN PACKAGE
12-LEAD (2mm × 2mm × 0.74mm)
TJMAX = 85°C, θJA = 92°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
TOP VIEW
12 11
5 6
OUT
VCC
ACPR
CHRG
STAT1
STAT2
BAT
ACIN
NTC
EN
PBEN
VSEL
10
9
8
7
1
2
3
4
13
GND
ORDER INFORMATION
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Input Voltage Range l2.7 5.5 V
VBAT Battery Voltage Range Charging 2.7 4.4 V
Not Charging, DC/DC On 3.1 4.4 V
IVCC VCC Quiescent Current Charging Done, DC/DC Off, VNTC > VDIS 50 80 µA
Charging Done, DC/DC Off, VNTC < VDIS 42 70 µA
IBATQ BAT Quiescent Current Charging Done, DC/DC Off, VBAT = 4.4V 4 8 µA
VACIN = VCC = 0, DC/DC On, IOUT = 0 37 75 µA
VACIN = VCC = 0, DC/DC Off 5 10 µA
VACIN = VCC = 0, Battery Disconnected
(VBAT < VDISCONNECT)
0 0.1 µA
The l denotes the specifications which apply over the specified operating
temperature range, otherwise specifications are at TA = 25°C (Notes 2, 3). VACIN = VCC = 5V, VBAT = 3.8V, unless otherwise noted.
TAPE AND REEL
PART NUMBER PART MARKING* FINISH CODE PAD FINISH
PACKAGE**
TYPE
MSL
RATING TEMPERATURE RANGE
LTC4126EV#TRPBF LHCP e4 Au (RoHS) LQFN (Laminate Package
with QFN Footprint) 3 –20°C to 85°C
Contact the factory for parts specified with wider operating temperature ranges. *Device temperature grade is identified by a label on the shipping container.
Parts ending with PBF are RoHS and WEEE compliant. **The LTC4126 package dimension is 2mm × 2mm × 0.74mm compared to a standard QFN
package dimension of 2mm × 2mm × 0.75mm.
This product only available in tape and reel or in mini-reel.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
LTC4126
3
Rev. A
For more information www.analog.com
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Notes 2, 3). VACIN = VCC = 5V, VBAT = 3.8V, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
AC Rectification
VCC(HIGH) VCC High Voltage Limit VCC Rising 5.25 5.5 5.75 V
VCC(LOW) VCC Low Voltage Limit VCC Falling 4.75 5 5.25 V
ACIN to VCC Voltage Drop 7.5mA from ACIN to VCC 0.6 V
Battery Charger
VCHG Battery Charge Voltage VSEL = 0
VSEL = 1
l
l
4.158
4.306
4.200
4.350
4.242
4.394
V
V
ICHG Battery Charge Current
l
7.0
6.5
7.5
7.5
8.0
8.5
mA
mA
ΔVUVLO VCC-to-VBAT Differential Undervoltage Lockout
Threshold (Indicated at ACPR Pin)
VCC Falling
VCC Rising
9
55
27
80
45
105
mV
mV
ΔVUVCL VCC-to-VBAT Differential Undervoltage Current
Limit Threshold Voltage
IBAT = 0.9 • ICHG
IBAT = 0.1 • ICHG
150
120
mV
mV
IDUVCL Charge Current Threshold for DUVCL Fault
Indication
(VCC – VBAT) Falling
(VCC – VBAT) Rising
3.1
4.5
mA
mA
VRECHRG Recharge Battery Threshold Voltage As a Percentage of VCHG 96.5 97.5 98.5 %
tTERMINATE Safety Timer Termination Period Timer Starts at the Beginning of the
Charge Cycle, VCC > (VBAT + 100mV)
5.1 6 6.9 hours
fSLOW Slow Blink Frequency 1.14 Hz
fFAST Fast Blink Frequency 4.58 Hz
VCOLD Cold Temperature Fault Threshold Voltage Rising Threshold Voltage 75.0 76.5 78 %VCC
Hysteresis 1.5 %VCC
VHOT Hot Temperature Fault Threshold Voltage Falling Threshold Voltage 33.4 34.9 36.4 %VCC
Hysteresis 1.5 %VCC
VDIS NTC Disable Threshold Voltage 150 250 mV
INTC NTC Leakage Current VNTC = 2.5V –100 100 nA
VNTC = 0V –150 nA
Step-Down DC/DC Regulator
VOUT DC/DC Regulator Output Voltage VBAT > VLOBAT1 or VDISCONNECT < VBAT <
VLOBAT2, IOUT = 0
l1.16 1.2 1.24 V
VLOBAT2 < VBAT < VLOBAT1, IOUT = 0 VBAT/3 V
VLOBAT1 Low Battery Alert 1 Threshold VBAT Falling l3.52 3.6 3.68 V
Hysteresis 100 mV
VLOBAT2 Low Battery Alert 2 Threshold VBAT Falling l3.22 3.3 3.38 V
Hysteresis 100 mV
VLOBAT3 Low Battery Alert 3 Threshold VBAT Falling l3.12 3.2 3.28 V
Hysteresis 100 mV
VDISCONNECT
Low Battery Disconnect Threshold Voltage VBAT Falling l2.93 3.0 3.07 V
fSW DC/DC Switching Frequency 3:1 Mode (VBAT > VLOBAT2)
2:1 Mode (VBAT < VLOBAT2)
l
l
40
60
50
75
60
90
kHz
kHz
ROL Effective Open-Loop Output Resistance (Note 4) VBAT = 3.5V, IOUT = 3mA 4.6 6.5 Ω
ILIM OUT Current Limit VOUT = 0V 80 mA
LTC4126
4
Rev. A
For more information www.analog.com
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Notes 2, 3). VACIN = VCC = 5V, VBAT = 3.8V, unless otherwise noted.
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into pins are positive; all voltages are referenced to
GND unless otherwise noted.
Note 3: The LTC4126E is tested under conditions such that TJ ≈ TA.
The LTC4126E is guaranteed to meet performance specifications from
0°C to 85°C junction temperature. Specifications over the –20°C to
85°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
junction temperature (TJ in °C) is calculated from the ambient temperature
(TA, in °C) and power dissipation (PD, in watts) according to the formula:
TJ = TA + (PD • θJA),
where the package thermal impedance θJA = 92°C/W).
Note that the maximum ambient temperature consistent with these
specifications is determined by specific operating conditions in
conjunction with board layout, the rated package thermal resistance, and
other environmental factors.
Note 4: See DC/DC Converter in Operation section.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Pushbutton Pin (PBEN)
VIL Logic Low Input Voltage l0.4 V
VIH Logic High Input Voltage l1.1 V
RPU Pull-up Resistance to BAT VPBEN < VIL 4 MΩ
IIH Logic High Input Leakage VPBEN = VBAT 0 0.1 μA
tDBL Debounce Time Low 348 425 503 ms
tDBH Debounce Time High 23 43 63 ms
EN, VSEL Pins
VIL Logic Low Input Voltage l0.4 V
VIH Logic High Input Voltage l1.1 V
IIL Logic Low Input Leakage 0 1 μA
IIH Logic High Input Leakage 0 1 μA
Logic Output Pins (STAT1, STAT2, ACPR)
VOL Logic Low Output Voltage 100μA into Pin 0.2 V
VOH Logic High Output Voltage 25μA out of Pin VOUT – 0.2V V
Open Drain Output (CHRG)
Pin Leakage Current VCHRG = 5V 0 0.5 μA
Pin Pull-Down Current VCHRG = 400mV 200 300 450 μA
LTC4126
5
Rev. A
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Charge Current vs Battery Voltage
Charge Voltage vs Temperature
(VSEL = 0V)
Charge Voltage vs Temperature
(VSEL = 5V)
Li-Ion Battery Charge Profile
Charge Current vs VCC-to-VBAT
Differential Voltage Charge Current vs Temperature
CHRG Pin Current vs Temperature
VCC Quiescent Current vs
Temperature
ACIN and VCC Waveform when
Shunt Active
TA = 25°C, unless otherwise noted.
V
CC
= 5V
V
SEL
= 0V
V
SEL
= 5V
BAT
(V)
2.7
3
3.3
3.6
3.9
4.2
4.5
0
1
2
3
4
5
6
7
8
CHARGE CURRENT (mA)
4126 G01
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
4.15
4.16
4.17
4.18
4.19
4.20
4.21
4.22
4.23
CHARGE VOLTAGE (V)
4126 G09
CC
= 5V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
4.30
4.31
4.32
4.33
4.34
4.35
4.36
4.37
4.38
CHARGE VOLTAGE (V)
4126 G02
CC
= 5V
BAT
= 3.8V
CC
- V
BAT
(mV)
0
50
100
150
200
250
0
1
2
3
4
5
6
7
8
CHARGE CURRENT (mA)
4126 G04
CC
= 5V
BAT
= 4.5V
NTC ENABLED
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
44
46
48
50
52
54
I(V
CC
) (µA)
4126 G07
VCC
ACIN
–10
–6
–2
2
6
10
VOLTAGE (V)
4126 G08
TIME (100µs/DIV)
CC
= 5V
BAT
= 3.8V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
7.0
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
CHARGE CURRENT (mA)
4126 G05
V
CC
= 5V
CHARGING DONE
CHRG
PIN PULLED UP TO 5V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
270
275
280
285
290
295
300
305
310
CURRENT (µA)
4126 G06
VCC = 5V
VSEL = 0V
CHARGE CURRENT
BATTERY VOLTAGE
TIME (MIN)
0
50
100
150
200
250
300
350
400
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
CHARGE CURRENT (mA)
VBAT (V)
4126 G03
LTC4126
6
Rev. A
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
DC/DC Output Voltage vs Battery
Voltage
DC/DC Efficiency vs Battery
Voltage
DC/DC Output Voltage vs
Temperature
DC/DC Output Voltage vs Load
Current
Maximum DC/DC Output Current
vs Battery Voltage
DC/DC Effective Open-Loop Output
Resistance vs Temperature
TA = 25°C, unless otherwise noted.
REGULATED 3:1 MODE
MODE
MODE
I
OUT
= 1mA
I
OUT
= 2mA
I
OUT
= 3mA
BAT
(V)
3
3.3
3.6
3.9
4.2
4.5
1.00
1.02
1.04
1.06
1.08
1.10
1.12
1.14
1.16
1.18
1.20
V
OUT
(V)
4126 G10
OPEN
LOOP
3:1
REG.
2:1
BAT
= 3.8V
3:1 MODE
2:1 MODE
IOUT (mA)
0
10
20
30
40
50
60
70
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17
1.18
1.19
1.20
VOUT (V)
4126 G20
OUT
= 1.1V
TA = –20°C
TA = 25°C
TA = 85°C
BAT
(V)
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
30
35
40
45
50
55
60
65
70
75
80
I
OUT
(mA)
4126 G21
BAT
= 3.5V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
0
1
2
3
4
5
6
RESISTANCE (Ω)
4126 G15
I
OUT
= 1mA
I
OUT
= 2mA
I
OUT
= 3mA
VBAT (V)
3
3.3
3.6
3.9
4.2
4.5
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
4126 G11
REGULATED 3:1 MODE
MODE
MODE
OPEN
LOOP
3:1
REG.
2:1
BAT
= 3.8V
R
OUT
= 1.2k
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
1.190
1.192
1.194
1.196
1.198
1.200
1.202
1.204
1.206
V
OUT
(V)
4126 G12
LTC4126
7
Rev. A
For more information www.analog.com
DC/DC Switching Frequency vs
Battery Voltage
DC/DC Switching Frequency in
3:1 Mode vs Temperature
DC/DC Switching Frequency in
2:1 Mode vs Temperature
BAT No-Load Quiescent Current
(DC/DC On) vs Battery Voltage
BAT Quiescent Current
(DC/DC Off) vs Temperature
DC/DC Output Transient Response
to Load Step
3:1 STEP–DOWN MODE
2:1 MODE
3
3.3
3.6
3.9
4.2
4.5
0
15
30
45
60
75
90
FREQUENCY (kHz)
4126 G13
VBAT (V)
BAT
= 3.5V
BAT
= 3.8V
BAT
= 4.4V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
42
43
44
45
46
47
48
49
50
51
52
FREQUENCY (kHz)
4126 G14
BAT
= 3.1V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
66
67
68
69
70
71
72
73
74
75
76
FREQUENCY (kHz)
4126 G19
3:1 STEP-DOWN MODE
2:1 MODE
(REGULATED)
3:1 MODE
(OPEN
LOOP)
I
OUT
= 0
3
3.3
3.6
3.9
4.2
4.5
0
5
10
15
20
25
30
CURRENT (µA)
4126 G16
VBAT (V)
CC
= 0V
BAT
= 3.1V
BAT
= 3.8V
BAT
= 4.4V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
3.0
3.5
4.0
4.5
5.0
5.5
6.0
CURRENT (µA)
4126 G17
OUT
= 2.2µF
TIME (800µs/DIV)
–80
–60
–40
–20
0
20
40
–10
0
10
20
30
40
50
V
OUT
(AC-COUPLED) (mV)
LOAD CURRENT (mA)
4126 G18
VOUT
LOAD CURRENT
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
LTC4126
8
Rev. A
For more information www.analog.com
PIN FUNCTIONS
NTC (Pin 1): Thermistor Input. Connect a thermistor from
NTC to GND, and a bias resistor from VCC to NTC. The
voltage level on this pin determines if the battery tempera-
ture is safe for charging. The charge current and charge
timer are suspended if the thermistor indicates a tem-
perature that is unsafe for charging. Once the temperature
returns to the safe region, charging resumes. Ground the
NTC pin if temperature qualified charging is not needed.
EN (Pin 2): Digital Logic Input Pin to Enable the DC/DC
Converter. A minimum voltage of 1.1V enables the regula-
tor provided that the LTC4126 is not in battery discon-
nect mode (see Battery Disconnect/Ship Mode under
Operation section). A low voltage (0.4V max) disables
the regulator and allows the pushbutton to control it. If
only pushbutton control is desired, tie this pin to ground.
Tie this pin to BAT if the DC/DC needs to remain enabled
all the time. Do not leave this pin unconnected.
PBEN (Pin 3): Pushbutton toggle input pin to enable/
disable the DC/DC converter. Enabling of the regulator
can only occur if the LTC4126 is not in battery discon-
nect mode (see Battery Disconnect/Ship Mode under
Operation section). A weak internal pull-up forces PBEN
high when not driven. A normally open pushbutton is con-
nected from PBEN to ground to force a low state on this
pin when the button is pushed. However, the pushbutton
is ignored if the EN input is high. If the pushbutton func-
tion is not needed, leave this pin unconnected.
VSEL (Pin 4): Digital Logic Input Pin to Select the Battery
Charge Voltage. A logic high on this pin selects 4.35V and
a logic low on this pin selects 4.2V. Do not leave this pin
unconnected.
ACPR (Pin 5): Digital CMOS Logic Output Pin to indicate
if there is enough input power available to charge the bat-
tery. This pin goes high when the VCC-to-BAT differential
voltage rises above 80mV (typical) and goes low when the
differential voltage drops below 27mV (typical). The low
level of this pin is referenced to GND and the high level
is referenced to the OUT pin voltage. Consequently, this
indicator is not available if the DC/DC is disabled.
CHRG (Pin 6): Open-Drain Charge Status Output Pin. This
pin can be pulled up through a resistor and/or an LED to
indicate the status of the battery charger. This pin has four
possible states: slow blink to indicate charging, fast blink
to indicate a fault, pulled down to indicate charging done,
and high impedance to indicate no input power. To con-
serve power, the pull-down current is limited to 300µA.
ACIN (Pin 7): AC Input Voltage Pin. Connect the exter-
nal LC tank, which includes the receive coil, to this pin.
Connect this pin to ground when not used.
BAT (Pin 8): Battery Connection Pin. Connect a single-cell
Li-Ion battery to this pin. Whenever enough input power
(AC or DC) is available, the battery will be charged via this
pin. Additionally, the DC/DC Converter is powered from
the battery via this pin. To minimize the effect of switching
noise from the DC/DC converter on charger performance,
this pin should be decoupled with a 1µF capacitor to GND
if the DC/DC converter is enabled while charging.
STAT2 (Pin 9), STAT1 (Pin 10): Digital CMOS Logic
Status Output Pins. The low level of these pins is refer-
enced to GND and the high level is referenced to VOUT.
Consequently, these indicators are not available if the
DC/DC is disabled. These two pins together with ACPR
indicate the various charging states and fault conditions.
However, when no input power is available and the DC/DC
converter is enabled, these pins instead indicate the volt-
age level of the battery.
VCC (Pin 11): DC Input Voltage Pin. An internal diode is
connected from the ACIN pin (anode) to this pin (cath-
ode). When an AC voltage is present at the ACIN pin, the
voltage on this pin is the rectified AC voltage. When the
ACIN pin is not used (or shorted to ground), connect
this pin to a DC voltage source to provide power to the
LTC4126 and charge the battery.
OUT (Pin 12): DC/DC Converter Output Pin. This pin pro-
vides 1.2V to power hearing aid ASICs. A low ESR ceramic
capacitor of at least 2.2μF should be placed close to this
pin to stabilize the converter.
GND (Exposed Pad Pin 13): Ground Pin. The exposed pad
on the backside of the package must be soldered to the
PCB ground for a low-resistance electrical connection as
well as for optimum thermal performance.
LTC4126
9
Rev. A
For more information www.analog.com
BLOCK DIAGRAM
+
–
–
+
+
–
+
–
+
–
VCC
RECHARGE
TERMINATED
LOW_BAT_ALERTS
BAT_DISCONNECT
BAT
300µA
LOGIC
C. C./C. V.
CHARGER
CONTROL
AND
STATUS
LOGIC
BAT
Li-Ion
7.5mA
4.2V/
4.35V
GND
4126 BD
CHRG
OUT
ACIN RECTIFICATION AND INPUT
POWER CONTROL
MULTI-MODE
CHARGE PUMP
DC/DC
REGULATOR
0.975VCHG
3.3V
3.6V
3.2V
3.0V
DUVCL
DUVLO
80mV/
27mV
13
8
6
EN
BAT
4M
PUSH-
BUTTON
2
NTC
1
7
11
154mV
DC/DC
ENABLE LOGIC
PBEN
3
+
RBIAS
RNTC
VCC
OUT
1.2V 12
OUT
STAT1
10
STAT2
9
OUT
ACPR
5
VSEL
4
PUSHBUTTON
TIMER AND
DEBOUNCER
CLK
CLK
150kHz
OSCILLATOR CLK
EN
+
–
–
+
+
–
TOO COLD
TOO HOT
150mV NTC ENABLE
+–
+–
Figure1. LTC4126 Block Diagram
LTC4126
10
Rev. A
For more information www.analog.com
OPERATION
The LTC4126 is a low power battery charger with an integrated
step-down DC/DC converter designed to wirelessly charge
single-cell Li-Ion batteries and provide a 1.2V output suitable
for powering a hearing-aid ASIC. The part has three principal
circuit components: an AC power controller, a full-featured
linear battery charger, and a step-down DC/DC converter.
AC POWER CONTROLLER
A complete wireless power transfer system consists of
transmit circuitry with a transmit coil and receive circuitry
with a receive coil. The LTC4126 resides on the receiver
side, where an external parallel resonant LC tank con-
nected to the ACIN pin allows the part to receive power
wirelessly from an alternating magnetic field generated
by the transmit coil. The Rectification and Input Power
Control circuitry (Figure1) rectifies the AC voltage at the
ACIN pin and regulates that rectified voltage at the VCC
pin to less than VCC(HIGH) (typically 5.5V).
Operation without Wireless Power
The LTC4126 can be alternately powered by connecting a
DC voltage source to the VCC pin directly instead of receiv-
ing power wirelessly through the ACIN pin. Ground the
ACIN pin if a voltage supply is connected to VCC.
BATTERY CHARGER
The LTC4126 includes a full-featured constant-current
(CC)/constant-voltage (CV) linear battery charger with
automatic recharge, automatic termination by safety
timer, bad battery detection, and out-of-temperature-
range charge pausing. Charge current is internally fixed
at 7.5mA and the final charge voltage is pin-selectable via
the VSEL pin to either 4.2V or 4.35V.
As soon as the voltage at the VCC pin rises 80mV (typical)
above the BAT pin voltage, the charger attempts to charge
the battery and a new charge cycle is initiated. A 6-hour
charge termination timer starts at the beginning of this
new charge cycle. When the VCC-to-BAT differential volt-
age rises above 154mV (typical), the charger enters con-
stant-current (CC) mode and charges the battery at the full
rated current of 7.5mA. When the BAT pin approaches the
final charge voltage, the charger enters constant-voltage
(CV) mode and the charge current begins to drop. The
charge current continues to drop while the BAT pin volt-
age is maintained at the proper charge voltage. This state
of CC/CV charging is indicated by a slow blinking LED
(typically 1.14Hz) at the CHRG pin.
After the 6-hour charge termination timer expires, charg-
ing stops completely. Once the charge cycle terminates,
the LED at the CHRG pin stops blinking and assumes a
pull-down state. To start a new charge cycle, remove the
power source at ACIN or VCC and reapply it.
Automatic Recharge
After charging has terminated, the charger draws only 3.7µA
(typical) from the battery. If it remains in this state long
enough, the battery will eventually discharge. To ensure that
the battery is always topped off, a new charge cycle auto-
matically begins when the battery voltage falls below V
RECHRG
(typically 97.5% of the charge voltage). In the event that the
battery voltage falls below VRECHRG while the safety timer is
still running, the timer will not reset. This prevents the timer
from restarting every time the battery voltage dips below
VRECHRG during a charging cycle.
Bad Battery Fault
If the battery fails to reach a voltage above V
RECHRG
by the
end of a full charge cycle of 6 hours, the battery is deemed
faulty and the LED at the CHRG pin indicates this bad
battery fault condition by blinking fast (typically 4.58Hz).
Differential Undervoltage Lockout (DUVLO)
A differential undervoltage lockout circuit monitors the
differential voltage between VCC and BAT and disables
the charger if the VCC voltage falls to within 27mV (typical
ΔVUVLO) of the BAT voltage. This condition is indicated by
a low on the ACPR pin. Charging does not resume until
this difference increases to 80mV at which time the ACPR
pin transitions back high. The DC/DC must be enabled for
proper ACPR indication.
Differential Undervoltage Current Limit (DUVCL)
The LTC4126 charger also includes differential undervolt-
age current limiting (DUVCL) which gradually reduces
the charge current from the full 7.5mA towards zero as
LTC4126
11
Rev. A
For more information www.analog.com
OPERATION
the V
CC
-to-BAT differential voltage drops from approxi-
mately 154mV to 116mV. See the curve in the Typical
Performance Characteristics section. When the charge
current reaches approximately 3.1mA, the LED at the
CHRG pin blinks fast (typically 4.58Hz) to indicate the
DUVCL fault. In the reverse direction, when the charge cur-
rent reaches approximately 4.5mA, the LED at the CHRG
pin resumes slow blinking to indicate normal operation.
Due to the finite hysteresis of the DUVCL comparator, it
is possible under a very narrow region of coupling condi-
tions for the LTC4126 to alternate between slow blinking
and fast blinking. This behavior should be construed as
operation at near (but not 100%) full charge current.
The DUVCL feature is particularly useful in situations
where the wireless power available is limited. Without
DUVCL, if the magnetic coupling between the receive coil
and the transmit coil is low, DUVLO could be tripped if the
charger tried to provide the full charge current. DUVLO
forces the charge current to drop to zero instantly, allowing
the supply voltage to rise above the DUVLO threshold and
switch on the charger again. In the absence of DUVCL, this
oscillatory behavior would result in intermittent charging.
The DUVCL circuitry prevents this undesirable behavior by
gradually increasing or decreasing the charge current as
input power becomes more or less available.
Temperature Qualified Charging
The LTC4126 monitors the battery temperature during the
charging cycle by using a negative temperature coefficient
(NTC) thermistor, placed close and thermally coupled to
the battery pack. If the battery temperature moves out-
side a safe charging range, the IC suspends charging and
signals a fault condition via CHRG (blinks fast at 4.58Hz)
and the STAT pins until the temperature returns to the safe
charging range. The safe charging range is determined by
two comparators (Too Hot and Too Cold) that monitor the
voltage at the NTC pin as shown in the Block Diagram.
The rising threshold of the Too Cold comparator is set to
76.5% of VCC (VCOLD) and the falling threshold of the Too
Hot comparator is set to 34.9% of V
CC
(V
HOT
), each with a
hysteresis of 1.5% of VCC around the trip point to prevent
oscillation. If the battery charger pauses due to a tempera-
ture fault, the 6-hour termination timer also pauses until
the thermistor indicates a return to a safe temperature.
Grounding the NTC pin disables all NTC functionality.
Most Li-Ion battery manufacturers recommend a tem-
perature range of 0°C to 40°C as a safe charging range.
Charge Status Indication via CHRG, ACPR, and
STAT pins
The status of the battery charger is indicated via the open-
drain CHRG pin as well as by the logic pins STAT1, STAT2,
and ACPR according to Table1. Indication by the logic
pins is available only when the DC/DC is enabled.
Table1. Charger Status Indication
CHRG ACPR STAT1 STAT2 STATUS
Hi-Impedance 0 X X Not Charging, No Power,
STAT pins indicate Battery
Level (see Table2)
Pulled LOW 1 0 0 Done Charging
Blink Slow (1.14Hz) 1 0 1 Charging
Blink Fast (4.58Hz) 1 1 0 Temperature Fault/Bad
Battery
Blink Fast (4.58Hz) 1 1 1 Differential Undervoltage
Current Limit (DUVCL)
The open-drain CHRG pin has an internal 300µA (typical)
pull-down. An LED can be connected between this pin and
VCC to indicate the charging status and any fault condition
as indicated in the table above. The ACPR, STAT1, and
STAT2 pins are digital CMOS logic outputs that can be
interpreted by a microprocessor. The low level of these
three pins is referenced to GND and the high level is ref-
erenced to the OUT pin voltage (typically 1.2V). Hence
the status indication via these three pins is only available
if the DC/DC converter is turned on via the EN pin or the
push button. Status indication via the CHRG pin is always
available during charging.
DC/DC CONVERTER
To supply the system load from the battery to the OUT pin,
the LTC4126 contains a proprietary low-noise multi-mode
charge pump DC/DC converter which can be switched
on by applying a minimum voltage of 1.1V to the EN
pin or by pushing the pushbutton. The converter can be
active simultaneously with the charger. The switching fre-
quency of the charge pump is set to either 50kHz or 75kHz
depending on the mode of operation. This frequency is
chosen to keep any switching noise out of the audio band.
LTC4126
12
Rev. A
For more information www.analog.com
OPERATION
Figure2. DC/DC Converter Thevenin Equivalent Circuit
in Mode 2: 3-to-1 Step-Down
Figure3. VOUT vs Battery Voltage at IOUT = 0
Modes of Operation
The charge pump DC/DC converter has 3 modes of opera-
tion depending on the battery voltage. For VBAT > 3.6V, the
charge pump operates in 3:1 step-down mode (Mode 1)
and provides a regulated 1.2V output. In Mode 1, the
maximum output current that the DC/DC converter can
provide is limited by internal current limit circuitry to
approximately 65mA.
When the battery voltage is between 3.6V and 3.3V, the
charge pump still operates in 3:1 step-down mode, but it
can no longer maintain 1.2V regulation and provides one-
third of the battery voltage at its output (only at no load).
This is referred to as Mode 2. The Thevenin equivalent
circuit of the converter in Mode 2 is shown in Figure2,
where ROL is the effective open-loop output resistance of
DC/DC CONVERTER
ROL
I
OUT
VOUT
4126 F02
+
–
+
–
VBAT
3
HEARING
AID ASIC
BATTERY VOLTAGE (V)
V
OUT
(V)
1.4
1.2
0.8
0.4
1.0
0.6
0.2
03.0 3.9
4126 F03
3.3 4.2
4.35
3.6
MODE 3
MODE 2
MODE 1
the converter. ROL is typically 4.6Ω at room temperature
for V
BAT
= 3.5V and f
SW
= 50kHz. It varies with the battery
voltage, the switching frequency of the converter, and the
temperature of the die. Figure2 can be used to determine
the output voltage (V
OUT
) for a specific load current (I
OUT
)
using the following equation:
VOUT =VBAT
3
– IOUT • ROL
When the battery voltage falls below 3.3V, the charge
pump switches to 2:1 step-down mode (Mode 3) and
again provides a regulated 1.2V output. In Mode 3, the
maximum output current that the DC/DC converter can
provide decreases with battery voltage but does not fall
below approximately 35mA. See the curve in the Typical
Performance Characteristics. The variation of the output
voltage versus the battery voltage for the various modes
of operation is shown in Figure3.
Handling Large Load
While operating in Mode 1 or Mode 2 (3:1 step-down
mode), if a large load at the output causes the output voltage
to drop below 1.1V, the converter automatically switches
over to Mode 3 (2:1 step-down mode) and attempts to
regulate the output at 1.2V. The converter stays in Mode 3
for 110ms (typical) and then returns to the previous mode.
If the large load condition persists and VOUT drops below
1.1V again, the converter switches back into Mode 3 for
another 110ms and the cycle continues. The duration of
110ms is chosen to prevent mode switching at a frequency
which could fall into the audible range. The switch over to
Mode 3 provides more current drive capability at the cost
of efficiency and this is why the converter tries to stay in
Mode 1 or Mode 2 as much as possible.
Converter Efficiency
The LTC4126 DC/DC converter efficiency varies through-
out the battery voltage range and is very much dependent
on the mode it is operating in. The theoretical maximum
efficiency in Mode 1 can be expressed as follows:
Efficiency, ηMode1 =VOUT
VBAT
3
⎛
⎝
⎜⎞
⎠
⎟
If regulation is maintained at the OUT pin at 1.2V, the
theoretical maximum efficiency is 85.7% when V
BAT
=
4.2V and 100% when VBAT = 3.6V as calculated from the
above equation.
When the battery voltage is between 3.6V and 3.3V, the
converter can no longer maintain a 1.2V regulation at OUT
at all loads and is operating in Mode 2. However, the upper
LTC4126
13
Rev. A
For more information www.analog.com
OPERATION
limit on the efficiency that the converter can achieve in this
mode is determined by switching losses, ohmic losses,
and quiescent current loss.
When the battery voltage falls to 3.3V, the converter
enters Mode 3 where the theoretical maximum efficiency
can be expressed as follows:
Efficiency, ηMode3 =
V
OUT
VBAT
2
⎛
⎝
⎜⎞
⎠
⎟
In Mode 3, the theoretical maximum efficiency is 72.7%
when VBAT = 3.3V and 80% when VBAT = 3.0V as calcu-
lated from the above equation.
Figure4 shows graphically the variation of the theoretical
maximum efficiency of the converter over the range of
battery voltages in the three different modes of operation.
Battery Disconnect/Ship Mode
When no input power is available and the battery voltage
falls to 3.0V (typical), the LTC4126 shuts down most of its
functions to prevent the battery from discharging too deeply,
consuming less than 100nA from the battery. Once in battery
disconnect mode, normal functioning can only resume when
power is applied to the ACIN or VCC pin and the VCC pin volt-
age rises 80mV (typical) above the BAT pin voltage.
The LTC4126 is also in battery disconnect mode after
initial installation of the battery regardless of its voltage
level. This implements the ship mode functionality.
Pushbutton Control
The LTC4126 is equipped with a pushbutton controller
to turn the DC/DC converter on and off if the EN pin is
not used (held low). A logic high on the EN pin overrides
the pushbutton function and keeps the regulator on. On
the falling edge of the EN signal, the DC/DC shuts off and
1µs later, the pushbutton can control the output as long
as EN remains low. A push on the pushbutton is consid-
ered valid if the PBEN pin is held low for at least 425ms
(typical). Additionally, the PBEN pin needs to return to the
high state for at least 43ms (typical) in between succes-
sive pushes for a push to be considered valid. An invalid
push will not change the state of the converter. A 4MΩ
internal resistor pulls up the PBEN pin to the BAT voltage.
A few different scenarios of valid and invalid pushes are
illustrated in Figure 5.
Figure5. Various Pushbutton Scenarios
VOUT
TOO SHORT LONG ENOUGH
425ms
43ms
LONG ENOUGH
(c) 1ST PUSH TOO SHORT, DC/DC STAYS OFF, 2ND PUSH VALID, DC/DC TURNS ON
PBEN
4126 F05
VOUT
LONG ENOUGH LONG ENOUGH
425ms 43ms
TOO SHORT
(b) HIGH PULSE TOO SHORT, 2ND PUSH IGNORED, DC/DC STAYS ON
PBEN
VOUT
LONG ENOUGH LONG ENOUGH
425ms
43ms
LONG ENOUGH
(a) VALID SUCCESSIVE PUSH, DC/DC TURNS ON AND OFF
PBEN
425ms
Figure4. Theoretical Maximum Converter
Efficiency vs Battery Voltage
BATTERY VOLTAGE (V)
EFFICIENCY (%)
100
90
70
50
40
30
20
80
60
10
04.2
4.35
3.0 3.9
4126 F04
3.3 3.6
MODE 3
MODE 2
MODE 1
Battery Level Indicator
The LTC4126 is equipped with a battery voltage monitor
which reports various battery voltage levels via the STAT
pins when not charging and the converter is enabled. See
Table2. Since the STAT pins indicate either the charger
status or the battery levels based on the state of ACPR,
there may be a delay of up to 1µs before the STAT pins
are valid whenever ACPR changes state.
Table2. Battery Level Indication
ACPR STAT1 STAT2 STATUS
0 0 0 VBAT < 3.2V, Low Battery Alert 3
0 0 1 3.2V < VBAT < 3.3V
0 1 0 3.3V < VBAT < 3.6V
0 1 1 VBAT > 3.6V
1 X X Power Available, STAT Pins Indicate Charger Status
LTC4126
14
Rev. A
For more information www.analog.com
Figure7. DC/AC Converter, Transmit/Receive Coil, Tuned Resonant LTC4126 Receiver (See
Table3 and Table4 for Recommended Components)
CRX
33nF
COUT
2.2µF
1.2V
ACIN
OUTNTC
GND
Li-Ion
4.35V
LTC4126
BAT
EN
CHRG
VCC
+
ACPR
STAT2
D1
U1
DIGITAL I/O
STAT1
VSEL
4126 F07
RECEIVERTRANSMITTER
PUSHBUTTON
PBEN
RSET
205k
C1
4.7µF
VIN
5V C2
100µF
LTX
7.5µH
LRX
13µH
GND
V+
OUT
fDRIVE = 244kHz
fLC_TANK = 315kHz
CTX1
33nF
CTX2
1nF
M1
Si2312CDS
SET
DIV
OE
LTC6990
AIR GAP
(2mm TO 4mm)
APPLICATIONS INFORMATION
Figure6. Wireless Power Transfer System
WIRELESS POWER TRANSFER
In a wireless power transfer system, power is transmitted
using an alternating magnetic field. An AC current in the
transmit coil generates a magnetic field. When the receive
coil is placed in this field, an AC current is induced in the
receive coil. The AC current induced in the receive coil is
a function of the applied AC current at the transmitter and
the magnetic coupling between the transmit and receive
coils. The LTC4126 internal diode rectifies the AC voltage
at the ACIN pin.
The power transmission range across the air gap as
shown in Figure6 can be improved using resonance by
connecting an LC tank to the ACIN pin tuned to the same
frequency as the transmit coil AC current frequency.
RECEIVER AND SINGLE TRANSISTOR TRANSMITTER
The single transistor transmitter shown in Figure7 is an
example of a DC/AC converter that can be used to drive
AC current into a transmit coil, LTX.
The NMOS, M1, is driven by a 50% duty cycle square
wave generated by the LTC6990 oscillator. During the
first half of the cycle, M1 is switched on and the current
through LTX rises linearly. During the second half of the
cycle, M1 is switched off and the current through LTX cir-
culates through the LC tank formed by C
TX
(= C
TX1
+ C
TX2
)
and LTX. The current through LTX is shown in Figure8.
If the transmit LC tank frequency is set to 1.29 times the
driving frequency, switching losses in M1 are significantly
reduced due to zero voltage switching (ZVS). Figure9 and
Figure10 illustrate the ZVS condition at different fTX-TANK
frequencies.
fTX-TANK = 1.29 • fDRIVE
fDRIVE is set by resistor RSET connected to the LTC6990.
fTX-TANK is set by:
fTX−TANK =1
2 • πLTX • CTX
The peak voltage of the transmit coil, LTX, that appears at
the drain of M1 is:
VTX-PEAK = 1.038 • π • VIN
And the peak current through LTX is:
ITX−PEAK =0.36 • VIN
f
TX−TANK
• L
TX
The RMS current through LTX is:
ITX-RMS = 0.66 • ITX-PEAK
4126 F06
LTX LRX
I
AC-TX I
AC-RX
AIR GAP
1:n
LTC4126
15
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
Note that since fDRIVE can be easily adjusted, it is best
practice to choose fRX-TANK using the minimum compo-
nent count (i.e. CRX) and then adjust fDRIVE to match.
The amount of AC current in the transmit coil can be
increased by increasing the supply voltage (VIN). Since
the amount of power transmitted is proportional to the AC
current in the transmit coil, VIN can be varied to adjust the
power delivery to the receive coil.
The overall power transfer efficiency is also dependent
on the quality factor (Q) of the components used in the
transmitter and receiver circuitry. Select components with
low resistance for transmit/receive coils and capacitors.
CHOOSING TRANSMIT POWER LEVEL
As discussed in the previous section, the supply voltage
(VIN) can be used to adjust the transmit power of the
transmitter shown in Figure7. Transmit power should be
set as low as possible to receive the desired output power
under worst-case coupling conditions (e.g. maximum
transmit distance with the worst-case misalignment).
Although the LTC4126 is able to shunt excess received
power to maintain the VCC voltage in the desired range, it
has the adverse effect of raising the die temperature and
possibly the battery temperature, and if the battery tem-
perature exceeds the Too Hot temperature threshold set
by the thermistor, the charger pauses charging the battery.
Using the rated current of the transmit inductor to set an
upper limit, transmit power should be adjusted down-
ward until charge current is negatively impacted under
worst-case coupling conditions. Once the transmit power
level is determined, the transmit and receive coils should
be arranged under best-case coupling conditions with a
fully-charged battery or a battery simulator to make sure
that the shunting of excess power does not raise the die
temperature too much.
In addition to temperature, another parameter that needs
to be checked is the maximum negative voltage on the
ACIN pin. Following the procedure above, when evaluating
the rise in temperature of the LTC4126 under the best-
case coupling conditions, ensure that VCC–VACIN does
not exceed 16V. Figure11 shows a typical waveform on
ACIN showing VCC–VACIN<16V.
Figure8. Current Through Transmit Coil
Figure9. Voltage on the Drain and Gate of NMOS
M1 when fTX_TANK = fDRIVE
Figure10. Voltage on the Drain and Gate of NMOS
M1 when fTX_TANK = 1.29 • fDRIVE
2µs/DIV
0A
4126 F08
500mA/DIV
2µs/DIV
0V
0V
GATE
VOLTAGE
2V/DIV
4126 F09
DRAIN
VOLTAGE
5V/DIV
2µs/DIV
0V
DRAIN
VOLTAGE
5V/DIV
GATE
VOLTAGE
2V/DIV
0V
4126 F10
The LC tank at the receiver, LRX and CRX, is tuned to the
same frequency as the driving frequency of the transmit
LC tank:
fRX-TANK = fDRIVE
where fRX-TANK is given by,
fRX−TANK =
1
2 • πLRX • CRX
LTC4126
16
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
resonant capacitance and RL-AC is the equivalent AC load
resistance.
One simplification is as follows:
RL–AC ≈
R
L–DC
2
which assumes that the drop across the Schottky diode is
much smaller than the amplitude |VRX|. Additionally, RL-DC
can be approximated as the ratio of the output voltage
(VOUT) to the output current (IOUT):
RL–DC =
V
OUT
I
OUT
The amplitude of the current in the transmit coil |ITX| can
be either measured directly or its initial (no receiver) value
can be calculated based on the transmitter circuit. This
initial value is a conservative estimate since the amplitude
of the transmitter coil current will drop as soon as the
receiver, with a load, is coupled to it.
The coupling factor (k) between the two coils could be
obtained by running a finite element simulation input-
ting the coil dimensions and physical configurations. An
easier method to obtain this coupling number, is to use the
series-aiding and series-cancelling measurement method
for two loosely coupled coils as shown in Figure13.
And:
L
AIDING =
L
AB
LCANCELLING =LCD
k=LAIDING – LCANCELLING
4 LTXLRX
Figure11. Typical Acceptable Voltage Waveform
on the ACIN Pin with VCC – VACIN < 16V.
Figure12. Modeling Parallel Resonant Configuration
and Half Wave Rectifier on the Receiver
Figure13. Series-Aiding and Series-Cancelling Method
Configurations Used for Measuring the Coupling Factor k
As an alternative to using the empirical method to deter-
mine the maximum negative voltage on the ACIN pin,
the following formula can be used in conjunction with
Figure12, which shows a parallel resonant configuration
on the receiver:
VRX =ωk LTXLRX
1– ω2LRXCRX
( )
2+ ω LRX
RL–AC
⎛
⎝
⎜
⎞
⎠
⎟
ITX
VCC
ACIN
–10
–6
–2
2
6
10
VOLTAGE (V)
4126 F11
TIME (100µs/DIV)
4126 F12
LRX CRX RL–AC
LTX
I
TX
VRX
LRX CRX CRECT R
L–DC
LTX
I
TX IOUT
V
OUT
•
•
•
•
4126 F13
LTX
LTX LRX
LRX
A
B C
D
|VRX| is the amplitude of the voltage on the receiver coil,
|ITX| is the amplitude of the current in the transmit coil,
k is the coupling factor between the transmit and receive
coils, is the operating frequency in radians per second,
LTX is the self-inductance of the transmit coil, LRX is the
self-inductance of the receive coil, CRX is the receiver
LTC4126
17
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
SINGLE TRANSISTOR TRANSMITTER AND LTC4126
RECEIVER-DESIGN EXAMPLE
The example in Figure7 illustrates the design of the reso-
nant coupled single transistor transmitter and LTC4126
charger. The steps needed to complete the design are
reviewed as follows.
1. Determine the receiver resonant frequency and set
component values for the receiver LC tank:
It is best practice to select a resonant frequency
that yields a low component count. In this example,
244kHz is selected as the receiver resonant frequency.
At 244kHz, the tank capacitance (CRX) required with
the selected receive coil (13µH) is 33nF. Since 33nF is
a standard value for capacitors, the tank capacitance
requires only one component. The tank capacitance
calculation is shown below.
CRX =
1
4 • π2• f2
RX−TANK
• L
RX
=32.7nF 33nF
Select a 33nF capacitor with a minimum voltage rating
of 25V and 5% (or better) tolerance for CRX. A higher
voltage rating usually corresponds to a higher quality
factor which is preferable. However, the higher the volt-
age rating, the larger the package size usually is.
2. Set the driving frequency (fDRIVE) for the single transis-
tor transmitter:
fDRIVE is set to the same value as the receiver resonant
frequency:
RSET =
1MHz
N
DIV
•
50k
Ω
244kHz =205kΩ
where NDIV = 1 as the DIV pin of the LTC6990 is
grounded. Select a 205kΩ (standard value) resistor
with 1% tolerance. For more information regarding the
oscillator, consult the LTC6990 data sheet.
3. Set the LC tank component values for the single tran-
sistor transmitter: If fDRIVE is 244kHz, the transmit LC
tank frequency (fTX-TANK) is:
fTX-TANK = 1.29 • 244kHz = 315kHz
The transmit coil (LTX) used in the example is 7.5µH.
The value of transmit tank capacitance (C
TX
) can be
calculated:
CTX =
1
4 • π2• f2
TX−TANK
• L
TX
=34nF
Since 34nF is not a standard capacitor value, use a
33nF capacitor in parallel with a 1nF capacitor to obtain
a value within 1% of the calculated CTX. The recom-
mended rating for CTX capacitors is 50V with 5% (or
better) tolerance.
4. Verify that the AC current through the transmit coil
is well within its rating. In this example, the supply
voltage to the single transistor transmitter is 5V. The
peak AC current through the transmit (LTX) coil can be
calculated as:
ITX –PEAK =
0.36 • V
IN
f
TX–TANK
• L
TX
=
0.36 • 5V
315kHz • 7.5µH =0.76A
and the RMS current as:
ITX-RMS = 0.66 • 0.76A = 0.5A
The rated current for the transmit coil is 1.55A (see the
Wurth 760308103206 data sheet for more informa-
tion). So the ITX–RMS calculated is well below the rated
current.
5. Also verify that the transmit power level chosen does
not result in excessive heating of the LTC4126.
COMPONENT SELECTION FOR TRANSMITTER AND
RECEIVER
To ensure optimum performance from the LTC4126,
use the components listed in Table3 and Table4 for
the receiver and transmitter, respectively, as shown in
Figure7. Select receive and transmit coils with good
quality factors to improve the overall power transmis-
sion efficiency. Use a ferrite core to improve the magnetic
coupling between the transmit and receive coils and to
shield the rest of the transmit and receive circuitry from
the AC magnetic field. Capacitors with low ESR and low
thermal coefficients such as C0G ceramics should be used
in the transmit and receive LC tanks.
LTC4126
18
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
COMPONENT SELECTION FOR CHRG STATUS
INDICATOR
The LED connected at the CHRG pin is powered by a
300µA (typical) pull-down current source. Select a high
efficiency LED with a low forward voltage drop. Some
recommended LEDs are shown in Table5.
Table5. Recommended LEDs
MANUFACTURER/
PART NUMBER PART DESCRIPTION
Rohm Semiconductor, SML-311UTT86 LED, 630nm, RED, 0603, SMD
Lite-On Inc. LTST-C193KRKT-5A LED, RED, SMT, 0603
Temperature Qualified Charging
To use the battery temperature qualified charging feature,
connect an NTC thermistor, RNTC, between the NTC pin
and GND, and a bias resistor, RBIAS, from the VCC pin to
the NTC pin (Figure14). Since the Too Hot comparator
threshold in the LTC4126 is internally set to 34.9% of VCC,
the resistance of the thermistor at the hot threshold, R
HOT
,
can be computed using the following equation:
R
HOT
R
HOT
+R
BIAS
=0.349
This can be simplified as:
RHOT
R
BIAS
=0.536
If RBIAS is chosen to have a value equal to the value of
the chosen NTC thermistor at 25°C (R25), then RHOT/R25
= 0.536. Thermistor manufacturers usually publish resis-
tance/temperature conversion tables for their thermistors
and list the ratio of the resistance, R
T
, of the thermistor at
any given temperature, T, to its resistance, R25, at 25°C.
For the Vishay thermistor NTCS0402E3104*HT with
β25/85 = 3950k, the ratio R
T
/R
25
= 0.536 corresponds
to approximately 40°C.
Figure14. NTC Thermistor Connection
LTC4126
NTC
BAT
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY
VCC
4126 F14
+
RBIAS
R
NTC
Li-Ion
Table3. Recommended Components for the Receiver Shown in Figure7
ITEM PART DESCRIPTION MANUFACTURER/PART NUMBER
LRX 13µH, 10mm, Receive Coil Wurth 760308101208
CRX Capacitor, C0G, 33nF, ±5%, 50V, 0805 or TDK C2012C0G1H333J125AA
Capacitor, C0G, 33nF, ±1%, 50V, 1206 Murata GCM3195C1H333FA16D
COUT Ceramic CAP, 2.2µF, ±10%, 6.3V, 0402 Murata GRM155R60J225KE95D
D1 LED, 630nm, Red, 0603, SMD Rohm Semiconductor SML-311UTT86
Table4. Recommended Components for the Transmitter Shown in Figure7
ITEM PART DESCRIPTION MANUFACTURER/PART NUMBER
LTX 7.5µH, 28mm × 15mm, Transmit Coil Wurth 760308103206
CTX1 Capacitor, C0G, 33nF, ±5%, 50V, 0805 TDK C2012C0G1H333J125AA
CTX2 Capacitor, C0G, 1nF, ±5%, 50V, 0603 TDK C1608C0G1H102J080AA
M1 MOSFET, N-CH 20V, 6A, SOT-23-3 Vishay Si2312CDS-T1-GE3
RSET Resistor, 205kΩ, ±1%, 1/16W, 0402 Vishay CRCW0402205KFKED
U1 IC, Voltage Controlled Silicon Oscillator, 2mm × 3mm DFN Analog Devices LTC6990IDCB
C1 Capacitor, X5R, 4.7μF, ±20%, 6.3V, 0402 TDK C1005X5R0J475M
C2 Capacitor, X5R, 100μF, ±20%, 6.3V, 1206 Murata GRM31CR60J107ME39L
LTC4126
19
Rev. A
For more information www.analog.com
Figure15. NTC Thermistor Connection with
Desensitizing Resistor RD
LTC4126
NTC
BAT
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY
VCC
4126 F15
RBIAS
RD
R
NTC
+
Li-Ion
APPLICATIONS INFORMATION
Similarly, since the Too Cold comparator threshold in the
LTC4126 is internally set to 76.5% of VCC, the resistance
of the thermistor at the cold threshold, RCOLD, can be
computed using the following equation:
RCOLD
R
COLD
+R
BIAS
=0.765
This can be simplified as:
RCOLD
R
BIAS
=3.25
Again, if RBIAS is chosen to have a value equal to the
value of the chosen NTC thermistor at 25°C (R25), then
RCOLD/R25 = 3.25. For the same Vishay thermistor with
β25/85 = 3950k, the ratio RT/R25 = 3.25 corresponds to
approximately 0°C.
The hot/cold temperature thresholds can be increased or
decreased by choosing a BIAS resistor which is not the
same as R25. For example, if a hot temperature threshold
of 50°C is desired, consult the resistance/temperature
conversion table of the thermistor to find the ratio R50/
R25. For the same Vishay thermistor used above, this ratio
is 0.3631. Since RHOT/RBIAS = 0.536, RBIAS can be calcu-
lated as follows:
RBIAS =RHOT
0.536
=0.3631• R25
0.536
=0.677 • R25
This means: choose an RBIAS value which is 67.7% of the
value of the thermistor at 25°C to set the hot temperature
threshold to 50°C. However, this will automatically shift
the cold temperature threshold upward too. The cold tem-
perature threshold can be recalculated by computing the
RCOLD/R25 ratio as follows:
RCOLD
R
25
=RCOLD
R
BIAS
•RBIAS
R
25
=3.25 • 0.677 =2.202
From the conversion table, this ratio corresponds to about
8°C. Note that changing the value of RBIAS to be smaller
than R25 moves both the hot and cold thresholds higher.
Similarly, RBIAS with a value greater than R25 will move
both the hot and cold thresholds lower. Also note that
with only one degree of freedom (i.e. adjusting the value
of RBIAS), the user can only set either the cold or hot
threshold but not both.
It is possible to adjust the hot and cold threshold inde-
pendently by introducing another resistor as a second
degree of freedom (Figure15). The resistor RD in effect
reduces the sensitivity of the resistance between the NTC
pin and ground. Therefore, intuitively this resistor will
move the hot threshold to a hotter temperature and the
cold threshold to a colder temperature. The value of RBIAS
and RD can now be set according to the following formula:
RBIAS =RCOLD – RHOT
( )
2.714
R
D
=0.197 • R
COLD
– 1.197 • R
HOT
Note that this method can only be used to push the hot
and cold temperature thresholds apart from each other.
When using the formulas above, if the user finds that
a negative value is needed for RD, the two temperature
thresholds selected are too close to each other and a
higher sensitivity thermistor is needed. For example, this
method can be used to set the hot and cold thresholds
independently to 60°C and –5°C. Using the same Vishay
thermistor with β25/85 = 3950k whose nominal value at
25°C is 100k, the formula results in R
BIAS
= 147k and
RD = 52.3k for the closest 1% resistors values.
LTC4126
20
Rev. A
For more information www.analog.com
APPLICATIONS INFORMATION
PC BOARD LAYOUT CONSIDERATIONS
Since the exposed pad of the LTC4126 package is the
only ground pin and serves as the return path for both
the charger and the DC/DC converter, it must be soldered
to the PC board ground for a good electrical connec-
tion. Although the LTC4126 is a low power IC, the shunt
circuitry in the AC power control block can cause a fair
amount of on-chip power dissipation if the available AC
power is excessive. If the heat is not dissipated properly
on the PC board, the temperature of the die and subse-
quently, the temperature of the battery may rise above
the hot temperature threshold set by the NTC thermistor
causing the charger to pause charging. For optimum ther-
mal performance, there should be a group of vias directly
under the exposed pad on the backside leading directly
down to an internal ground plane. To minimize parasitic
inductance, the ground plane should be as close as pos-
sible to the top plane of the PC board (Layer 2).
LTC4126
21
Rev. A
For more information www.analog.com
Full-Featured Application Circuit
Minimum Component Count Application Circuit
TYPICAL APPLICATIONS
VIN
4.75V TO 5.25V
LTX
6.5µH
CTX
100nF
AIR GAP
(2mm TO 4mm)
CRX
68nF
2.2µF
1.2V
ACIN
OUT
VSEL
GND
Li-Ion
4.2V
LTC4126
BAT
NTC
EN
RBIAS
100k
D1
RNTC
CHRG
VCC
GND
µP
V+
+
ACPR
STAT2 GPIO
STAT1
PUSHBUTTON
NTC RESISTOR
THERMALLY
COUPLED
WITH BATTERY
PBEN
4126 TA02
LRX
8µH
CTX: TDK C3216COG2A104J160AC
LTX: WURTH 760-308-101-104
RNTC: VISHAY NTCS0402E3104*HT
M1: VISHAY Si2312CDS-T1-GE3
D1: ROHM SEMICONDUCTOR SML-311UTT86
GND
LTC4125
1.5M
FB
IN1IN2
NTC
CTD
CTS
PTHM
EN
FTH
DTH
IS–
IS+
IMON
PTH2
PTH1
IN
SW2
SW1
STAT
47µF
0.01µF
1.05k
20.5k
100k
100k
22mΩ
17.8k
47µF
V
IN
LTX LRX
8µH
CRX
68nF
2.2µF
1.2V
TRANSMITTER
AIR GAP
ACIN
OUTVSEL GND
Li-Ion
4.2V
4126 TA04
LTC4126
BAT
NTC
+
–
+
EN
CRX: AVX0603YC683JAT2A
L
RX
:
SUNLORD MQQRC060630S8R0
LTC4126
22
Rev. A
For more information www.analog.com
PACKAGE DESCRIPTION
LQFN Package
12-Lead (2mm × 2mm × 0.74mm)
(Reference LTC DWG # 05-08-1530 Rev B)
DETAIL B
A
PACKAGE TOP VIEW
5
PIN 1
CORNER
Y
X
aaa Z2×
12b
PACKAGE BOTTOM VIEW
4
6
SEE NOTES
E
D
b
e
e
b
E1
D1
DETAIL B
SUBSTRATE
MOLD
CAP
// bbb Z
Z
H2
H1
L
DETAIL A
DETAIL C
SUGGESTED PCB LAYOUT
TOP VIEW
0.0000
0.0000
0.7500
0.2500
0.2500
0.7500
0.2500
0.2500
DETAIL A
PIN 1 NOTCH
0.14 × 45°
11 12
6 5
1
4
10
7
aaa Z
2×
MX YZccc
MX YZccc
MX YZeee
MZfff
PACKAGE
OUTLINE
0.25 ±0.05
0.70 ±0.05
2.50 ±0.05
2.50 ±0.05
LQFN 12 0618 REV B
0.250
0.70
0.70
ddd Z
12×
Z
A1
DETAIL C
SYMBOL
A
A1
L
b
D
E
D1
E1
e
H1
H2
aaa
bbb
ccc
ddd
eee
fff
MIN
0.65
0.01
0.30
0.22
NOM
0.74
0.02
0.40
0.25
2.00
2.00
0.70
0.70
0.50
0.24 REF
0.50 REF
MAX
0.83
0.03
0.50
0.28
0.10
0.10
0.10
0.10
0.15
0.08
NOTES
DIMENSIONS
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS
3. PRIMARY DATUM -Z- IS SEATING PLANE
METAL FEATURES UNDER THE SOLDER MASK OPENING NOT SHOWN
SO AS NOT TO OBSCURE THESE TERMINALS AND HEAT FEATURES
5
4
DETAILS OF PIN 1 IDENTIFIER ARE OPTIONAL, BUT MUST BE
LOCATED WITHIN THE ZONE INDICATED. THE PIN 1 IDENTIFIER
MAY BE EITHER A MOLD OR MARKED FEATURE
6 THE EXPOSED HEAT FEATURE MAY HAVE OPTIONAL CORNER RADII
e
e/2
SUBSTRATE THK
MOLD CAP HT
LTC4126
23
Rev. A
For more information www.analog.com
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 02/20 Modified Battery Charger VCHG spec 3
LTC4126
24
Rev. A
For more information www.analog.com
ANALOG DEVICES, INC. 2018-2020
02/20
www.analog.com
RELATED PARTS
TYPICAL APPLICATION
CRX
68nF
2.2µF
1.2V
ACIN
OUT
GND
Li-Ion
4.35V
LTC4126
BAT
EN
CHRG
VCC
+
ACPR
STAT2 DIGITAL I/O
STAT1
NTC
VSEL
PUSHBUTTON
PBEN
4126 TA03
RBIAS
100k
D1
LRX
8µH
RNTC
NTC RESISTOR
THERMALLY
COUPLED
WITH BATTERY
R1
232k
C1
4.7µF
V
IN
5V C2
100µF
LTX
2.7µH
GND
V+
OUT
fDRIVE = 216kHz
fLC_TANK = 266kHz
CTX1
33nF
CTX2
100nF
M1
Si2312CDS
SET
MOD
DIV
LTC6992
AIR GAP
(2mm TO 4mm)
R2
732k
R3
66.5k
CTX1: TDK C2012COG1H333J125AA
CTX2: TDK C3216COG1H104J160AA
LTX: SUNLORD MQQTC202030S2R7
CRX: AVX0603YC683JAT2A
LRX: SUNLORD MQQRC060630S8R0
RNTC: VISHAY NTCS0402E3104*HT
M1: VISHAY Si2312CDS-T1-GE3
D1: ROHM SEMICONDUCTOR STL-311UTT86
Wireless 7.5mA Li-Ion Battery Charger (4.35V) Tuned at 266kHz with Pushbutton Enabling
PART NUMBER DESCRIPTION COMMENTS
LTC4120 400mA Wireless Power Receiver Buck Battery
Charger
Wireless 1 to 2 Cell Li-Ion Charger, 400mA Charge Current, Dynamic Harmonization
Control, Wide Input Range: 12.5V to 40V, 16-Lead 3mm × 3mm QFN Package
LTC4123 Low Power Wireless Charger for Hearing Aids Wireless Single NiMH Charger, Integrated Rectifier with Overvoltage Limit, 25mA Charge
Current, Zn-Air Detect, Temperature Compensated Charge Voltage, 6-Lead 2mm × 2mm
DFN Package
LTC4125 5W Auto Resonant Wireless Power Transmitter Monolithic Auto Resonant Full Bridge Driver. Transmit Power Automatically Adjusts to
Receiver Load, Foreign Object Detection, Wide Operating Switching Frequency Range:
50kHz to 250kHz, Input Voltage Range 3V to 5.5V, 20-Lead 4mm × 5mm QFN Package
LTC4070 Li-Ion/Polymer Shunt Battery Charger System Li-Ion/Polymer Shunt Battery Charger, Low Operating Current (450nA), 50mA Internal
Shunt Current, Pin Selectable Float Voltages (4.0V, 4.1V, 4.2V), 8-Lead 2mm × 3mm
DFN and MSOP Packages
LTC4071 Li-Ion/Polymer Shunt Battery Charger System
with Low Battery Disconnect
Charger Plus Pack Protection in One IC, Low Operating Current (550nA), 50mA Internal
Shunt Current, Pin Selectable Float Voltages (4.0V, 4.1V, 4.2V), 8-Lead 2mm × 3mm
DFN and MSOP Packages
LTC6990 TimerBlox: Voltage Controlled Silicon Oscillator Fixed-Frequency or Voltage-Controlled Operation, Frequency Range of 488Hz to 2MHz,
Low-Profile SOT-23 and 2mm × 3mm DFN Packages
LTC6992 TimerBlox: Voltage Controlled Pulse-Width
Modulator (PWM)
Pulse Width Modulation by 0V to 1V Analog Input, Frequency Range of 3.81Hz to 1MHz,
Low-Profile SOT-23 and 2mm × 3mm DFN Packages