LTC4123
1
4123fa
For more information www.linear.com/LTC4123
Typical applicaTion
FeaTures DescripTion
Low Power
Wireless Charger for Hearing Aids
The LT C
®
4123 is a low power wireless receiver and a
constant-current/constant-voltage linear charger for NiMH
batteries. An external programming resistor sets the charge
current up to 25mA. The temperature compensated charge
voltage feature protects the NiMH battery and prevents
overcharging.
Wireless charging with the LTC4123 allows products to
be charged while sealed within enclosures and eliminates
bulky connectors in space constrained environments. The
LTC4123 also makes it possible to charge NiMH batteries
used in moving or rotating equipment.
The LTC4123 prevents charging of Zinc-Air batteries as well
as batteries inserted with reverse polarity. The LTC4123
pauses charging if its temperature is too hot or too cold.
An internal timer provides time-based charging termination.
The 2mm × 2mm DFN package and low external compo-
nent count make the LTC4123 well-suited for hearing aid
applications or other low power portable devices where
small solution size is mandatory.
25mA NiMH Wireless Battery Charger
applicaTions
n Complete Low Power Wireless NiMH Charger
n Low Minimum Input Voltage: 2.2V
n Small Total Solution Volume
n 1.5V, 25mA Linear Single-Cell NiMH Charger
n Temperature Compensated Charge Voltage
n Integrated Rectifier with Overvoltage Limit
n Zinc-Air Battery Detection
n Reverse Polarity Protection
n Thermally Enhanced 6-Lead (2mm × 2mm)
DFN package
n Hearing Aids
n Smart Cards
n Fitness Devices
n Moving and/or Rotating Equipment
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Complete Wireless Charging Solution for a Hearing Aid
VIN
Tx COIL
LED
LRX
13µH
CRX
33nF
CIN
4.7µF RPROG
953Ω
TRANSMITTER
CIRCUIT
AIR GAP
ACIN
BAT
CHRG
GND PROG
1.5V
NiMH
BATTERY
4123 TA01
LTC4123
VCC
ICHARGE =
25mA MAX
+
–
+
LTC4123
2
4123fa
For more information www.linear.com/LTC4123
pin conFiguraTionabsoluTe MaxiMuM raTings
Input Supply Voltages
VCC........................................................ –0.3V to 5.5V
ACIN .................................................. –10V to VCC+1V
Input Supply Currents
I(ACIN) ............................................................ 200mA
BAT ................................................................. –2V to 2V
PROG, CHRG..................................... –0.3V to VCC+0.3V
Operating Junction Temperature Range
(Note 2) ........................................................ –20 to 85°C
Storage Temperature Range ......................–65 to 150°C
(Notes 1, 3)
TOP VIEW
GND
BAT
PROG
ACIN
VCC
CHRG
DC PACKAGE
6-LEAD (2mm × 2mm) PLASTIC DFN
4
5
7
GND
6
3
2
1
TJMAX = 85°C, θJA = 80.6°C/W
EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB
orDer inForMaTion
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Input Supply Operating Range l2.2 5 V
IVCC Input Quiescent Operating Current Charging Terminated. IBAT and IPROG = 0A l125 200 µA
VUVLO Input Supply Undervoltage Lockout Threshold VCC Rising 1.88 1.95 2.02 V
Hysteresis 40 mV
VBAT Battery Charge Voltage TA = 25°C 1.4955 1.5075 1.5195 V
TA = –10°C (Note 4) 1.580 1.595 1.610 V
TA = 75°C (Note 4) 1.3675 1.3825 1.3975 V
IBAT(LEAK) Battery Pin Discharge Current Charger Terminated or VCC < VUVLO, VBAT = 2V 100 nA
VPROG PROG Pin Servo Voltage 0.25 V
hPROG Ratio of BAT Current to PROG Current 96 mA/mA
ICHG Constant-Current Mode Charge Current RPROG = 23.7kΩ l0.73 1 1.27 mA
RPROG = 953Ω l22 25 28 mA
VUVCL Undervoltage Current Limit RPROG = 4.99kΩ 2.2 V
TCHG Charge Termination Period 4.8 6 7.2 Hours
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC4123EDC#PBF LTC4123EDC#TRPBF LGSY 6-Lead (2mm × 2mm) Plastic DFN –20°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
elecTrical characTerisTics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C. VACIN = 0V, VCC = 5V unless otherwise noted (Notes 2, 3, 4).
http://www.linear.com/product/LTC4123#orderinfo
LTC4123
3
4123fa
For more information www.linear.com/LTC4123
elecTrical characTerisTics
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: The LTC4123 is tested under conditions such that TJ ≈ TA. The
LTC4123E is guaranteed to meet specifications from 0°C to 85°C junction
temperature. Specifications over the –20°C to 85°C operating junction
temperature are assured by design, characterization and correlation with
statistical process controls. 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
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C. VACIN = 0V, VCC = 5V unless otherwise noted (Notes 2, 3, 4).
impedance and other environmental factors. The junction temperature
(TJ, in °C) is calculated from the ambient temperature (TA, in °C) and
power dissipation (PD, in Watts) according to the following formula:
TJ = TA + (PD • θJA), where θJA (in °C/W) is the package thermal
impedance.
Note 3: All currents into pins are positive; all voltages are referenced to
GND unless otherwise noted.
Note 4: These parameters are guaranteed by design and are not 100%
tested. The battery charge voltage variation over temperature is guaranteed
in a ±15mV band as shown in the Typical Performance Characteristics curve.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Thermal Sensing
Cold Temperature Fault Threshold Die Temperature Falling –5 °C
Hysteresis 5 °C
Hot Temperature Fault Threshold Die Temperature Rising 70 °C
Hysteresis 5 °C
Zinc-Air Battery Detection
VZn-AIR Zinc-Air Fault Threshold Voltage VBAT Rising 1.60 1.65 V
Hysteresis 40 mV
TZn-AIR Zinc-Air Detection Period 80 s
Charge Voltage Limit During Zinc-Air Battery Detection 1.8 V
Zinc-Air Detection Charge Current RPROG = 23.7kΩ 1 mA
Reverse Polarity Detection
VREVPOL Reverse Polarity Threshold Voltage VBAT Falling –50 mV
Hysteresis 40 mV
AC Rectification
VCC(HIGH) VCC High Voltage Limit VCC Rising 5 V
VCC(LOW) VCC Low Voltage Limit VCC Falling 3 V
ACIN to VCC Voltage Drop IVCC = –20mA, Charger Terminated 0.65 V
Status Pin (CHRG)
ICHRG CHRG Pin Pull-Down Current VCHRG = 450mV 250 340 430 µA
CHRG Leakage Current CHRG = 5V 1 µA
LTC4123
4
4123fa
For more information www.linear.com/LTC4123
Typical perForMance characTerisTics
PROG Pin Voltage vs Temperature
(Constant Current Mode)
Undervoltage Current Limit:
Charge Current vs Supply Voltage
Charge Current
vs PROG Pin Voltage
Input Quiescent Current
vs Supply Voltage
Battery Leakage Current
vs Temperature
UVLO Threshold vs Temperature
(Rising and Falling)
Battery Charge Current
vs Battery Charge Voltage
Battery Charge Voltage
vs Temperature
Battery Charge Voltage
vs Supply Voltage
TA = 25°C, unless otherwise noted.
R
PROG
= 2.49kΩ
V
BAT
(V)
1.30
1.35
1.40
1.45
1.50
1.55
1.60
0
2.0
4.0
6.0
8.0
10.0
12.0
I
CHG
(mA)
4123 G01
R
PROG
= 23.7kΩ
TEMPERATURE (°C)
–5
10
25
40
55
70
1.380
1.400
1.420
1.440
1.460
1.480
1.500
1.520
1.540
1.560
1.580
1.600
V
BAT
(V)
4123 G02
CHARGE VOLTAGE
CHARGE VOLTAGE MAX
CHARGE VOLTAGE MIN
R
PROG
= 23.7kΩ
SUPPLY VOLTAGE (V)
2.5
3
3.5
4
4.5
5
1.495
1.500
1.505
1.510
1.515
1.520
V
BAT
(V)
4123 G03
R
PROG
= 23.7k
TEMPERATURE (°C)
–5
10
25
40
55
70
240
245
250
255
260
V
PROG
(mV)
4123 G04
R
PROG
= 2.49kΩ
SUPPLY VOLTAGE (V)
2
2.2
2.4
2.6
2.8
3.0
0
2.0
4.0
6.0
8.0
10.0
12.0
I
CHG
(mA)
4123 G05
R
PROG
= 23.7kΩ
V
PROG
(mV)
0
50
100
150
200
250
0
0.20
0.40
0.60
0.80
1.00
I
CHG
(mA)
4123 G06
V
BAT
= –100mV
SUPPLY VOLTAGE (V)
2
2.5
3
3.5
4
4.5
5
100
110
120
130
140
150
I
VCC
(µA)
4123 G07
V
BAT
= 2V
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
–100
–80
–60
–40
–20
0
20
40
60
80
100
I
BAT(LEAK)
(nA)
4123 G08
V
CC
= 0V
UVLO FALLING
UVLO RISING
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
1.86
1.88
1.90
1.92
1.94
1.96
1.98
2.00
SUPPLY VOLTAGE (V)
4123 G09
LTC4123
5
4123fa
For more information www.linear.com/LTC4123
Typical perForMance characTerisTics
CHRG Pull-Down Current
vs Temperature
Charge Timer Accuracy
vs Supply Voltage
Maximum Available Wireless
Power vs Coil Spacing Typical Wireless Charging Cycle
Charge Termination Period
vs Temperature
VCC High and Low Thresholds
vs Temperature
TA = 25°C, unless otherwise noted.
V
CC(HIGH)
V
CC(LOW)
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
2.50
3.00
3.50
4.00
4.50
5.00
SUPPLY VOLTAGE (V)
4123 G10
TEMPERATURE (°C)
–20
–5
10
25
40
55
70
85
280
300
320
340
360
380
I
CHRG
(µA)
4123 G11
TEMPERATURE (°C)
–5
10
25
40
55
70
4.80
5.40
6.00
6.60
7.20
T
CHG
(Hours)
4123 G12
SUPPLY VOLTAGE (V)
2.2
2.9
3.6
4.3
5.0
–20.0
–15.0
–10.0
–5.0
0
5.0
10.0
15.0
20.0
CHARGE TIMER ACCURACY (%)
4123 G13
R
PROG(MIN)
= 953Ω
L
RX
= 760308101208
L
TX
= 760308103206
f
DRIVE
= 244kHz
See Figure 4
MAX POWER
MAX CHARGE CURRENT
COIL SPACING (mm)
1.5
3.5
5.5
7.5
9.5
11.5
0
25
50
75
100
125
0
6
12
18
24
30
MAXIMUM AVAILABLE POWER (mW)
MAXIMUM CHARGE CURRENT AVAILABLE (mA)
4123 G14
0
1
2
3
4
5
6
TIME (HOURS)
0
65
130
195
260
P
R
O
G
V
(
m
V
)
4123 G15
P
R
O
G
V
B
A
T
V
See Figure 4
P675 NiMH
P
R
O
G
R
=
9
7
6
Ω
D
R
I
V
E
f
=
2
4
4
k
H
z
T
X
L
=
7
6
0
3
0
8
1
0
3
2
0
6
R
X
L
=
7
6
0
3
0
8
1
0
1
2
0
8
0.0
0.4
0.8
1.2
1.6
B
A
T
V
(
V
)
LTC4123
6
4123fa
For more information www.linear.com/LTC4123
pin FuncTions
ACIN (Pin 1): AC Input Voltage. Connect the external LC
tank, which includes the receive inductor, to this pin. Short
this pin to ground when not used.
VCC (Pin 2): The DC input voltage range is 2.2V to 5V. An
internal diode is connected from the ACIN pin (anode) to
this pin (cathode). When an AC voltage is present at the
ACIN pin, the voltage on this pin is the rectified AC voltage.
Connect a 4.7µF capacitor to ground on this pin. When the
ACIN pin is not used (shorted to ground), connect this pin
to a DC voltage source to provide power to the part and
to charge the battery.
CHRG (Pin 3): Open-Drain charge status output. CHRG
requires a pull-up resistor and/or LED to indicate the status
of the battery charger. This pin has four possible states:
powered on/charging (blink slow), no power /not charging
(high impedance), charging complete (pull-down), and
Zinc-Air battery/reverse polarity detection/ battery tem-
perature out of range/UVCL at the beginning of the charge
cycle (blink fast). To conserve power, this pin implements
a 340µA pull-down current source.
PROG (Pin 4): The charge current program pin. A 1%
resistor, RPROG, connected from PROG to ground programs
the charge current. In constant-current charging mode,
the voltage at this pin is regulated to 0.25V. The voltage
on this pin sets the constant current charge current to:
ICHG =
96 •V
PROG
R
PROG
=
24V
R
PROG
BAT (Pin 5): Battery connection pin. Connect the NiMH
battery to this pin. At 25°C, the battery voltage is regulated
to 1.5075V. This charge voltage is temperature compen-
sated with a temperature coefficient of –2.5mV/ºC.
GND (Pin 6, Exposed Pad Pin 7): Ground. Connect the
ground pins to a suitable PCB copper ground plane for
proper electrical operation. The exposed pad must be
soldered to PCB ground for the rated thermal performance.
LTC4123
7
4123fa
For more information www.linear.com/LTC4123
block DiagraM
CHARGING
(SLOW BLINK)
CHARGING COMPLETE
(ON)
BAT FAULT
(BLINK FAST)
+
–
–
+
+
–
+
–
+
–
+
–
+
–
VCC
VUVLO
PROG
PROG
VPROG
VUVCL
VCC
TEMP FAULT
ZINC-AIR
BAT FAULT
REVERSE
POLARITY FAULT
340µA
LOGIC
BAT
BAT
BAT
+
GND 4123 BD
ACIN
CHRG
RECTIFICATION AND INPUT
POWER CONTROL
CONSTANT CURRENT (CC)
+
CONSTANT VOLTAGE (CV)
+
UNDERVOLTAGE CURRENT LIMIT
(UVCL)
VZn-AIR
TDIE
VCC
TREF
IBAT
96
VREVPOL
CC
BAT
CV
UVCL
NEGATIVE TC
VOLTAGE
REFERENCE
Figure 1. Block Diagram
LTC4123
8
4123fa
For more information www.linear.com/LTC4123
operaTion
The LTC4123 is a low power battery charger designed to
wirelessly charge single-cell NiMH batteries. The charger
uses a constant-current/constant-voltage charge algorithm
with a charge current programmable up to 25mA. The final
charge voltage is temperature compensated to reach an
optimum state-of-charge and prevent overcharging of the
battery. The LTC4123 also guarantees the accuracy of the
charge voltage to ±15mV from –5°C to 70˚C (see typical
performance characteristics).
An external resonant LC tank connected to the ACIN
pin allows the part to receive power wirelessly from an
alternating magnetic field generated by a transmit coil.
A complete wireless power transfer system consists
of transmit circuitry, with a transmit coil, and receive
circuitry, with a receive coil. The Rectification and Input
Power control circuitry (Figure 1) rectifies the AC voltage
at the ACIN pin and regulates the rectified voltage at VCC
to less than VCC(HIGH) (typically 5V).
An LED can be connected to the CHRG pin to indicate the
status of the charge cycle and any fault conditions. An
internal thermal limit will stop charging and pause the
6-hour charge timer if the die temperature rises above
70˚C or falls below –5˚C.
In a typical charge cycle (see Figure 2), the 6-hour charge
timer will begin when the part is powered. At the beginning
of the charge cycle, the LTC4123 will determine if the battery
is connected in reverse or if a Zinc-Air battery is connected
to the BAT pin. If any of the above fault conditions is true,
the BAT pin goes to a high impedance state and charging
is stopped immediately. An LED connected to CHRG will
blink fast (typically at 6Hz). If the battery is a NiMH battery
inserted with correct polarity, it will continue to charge at
the programmed current level in constant-current mode
and CHRG will blink slowly (typically at 0.8Hz).
When the BAT pin approaches the final charge voltage, the
LTC4123 enters constant-voltage mode and the charge
current begins to drop. The charge current will continue
to drop and the BAT pin voltage will be maintained at the
proper charge voltage. After the charge termination timer
expires, charge current ceases and the BAT pin assumes a
high impedance state. Once the charge cycle terminates,
the CHRG pin stops blinking and assumes a pull-down
state. To start a new charge cycle, remove the input volt-
age at ACIN or VCC and reapply it.
Input Voltage Qualification
An internal undervoltage lockout (UVLO) circuit monitors
the input voltage at VCC and disables the LTC4123 until
VCC rises above VUVLO (typically 1.95V). The UVLO circuit
has a built-in hysteresis of approximately 40mV. During
undervoltage conditions, maximum battery drain current
is IBAT(LEAK) (100nA maximum).
The LTC4123 also includes undervoltage current limiting
(UVCL) that prevents charging at the programmed current
until the input supply voltage is above VUVCL (typically 2.2V).
UVCL is particularly useful in situations when the wireless
power available is limited. Without UVCL if the magnetic
coupling between the receive coil and transmit coil is low,
UVLO could be easily tripped if the charger tries to provide
the full charge current. UVLO forces the charge current to
zero, which allows the supply voltage to rise above the
UVLO threshold and switch on the charger again. This
oscillatory behavior will result in intermittent charging. The
UVCL circuitry prevents this undesirable behavior.
Battery Fault Conditions
The LTC4123 detects the presence of Zinc-Air batteries at
the beginning of the charge cycle. Initially, the LTC4123
will charge the battery at full charge current and if the
BAT pin rises above VZn-AIR (typically 1.65V) in TZn-AIR
(typically 80 seconds) or less from the start of the charge
timer, the LTC4123 determines the battery connected is
a Zinc-Air battery and charging is disabled immediately.
The charging cycle continues normally otherwise. The
LTC4123
9
4123fa
For more information www.linear.com/LTC4123
operaTion
UNIT POWERED
START CHARGE TIMER
START CHARGING
PULSE LED SLOWLY
*REVERSE BATTERY CONDITION IS
CHECKED THROUGHOUT THE ALGORITHM
**IF THE DIE TEMPERATURE IS TOO HIGH
OR TOO LOW DURING ZINC-AIR BATTERY
DETECTION (80 SECONDS), THIS 80 SECOND
TIMER WILL BE RESET
*BAT < –50mV?
BAT > 1.65V?
TIME = 80sec?
**DIE TEMPERATURE
TOO HIGH OR
TOO LOW?
CHARGE TIMER
EXPIRED?
NO
NO
NO
NO
NO
YES
YES
YES
4123 F02
YES
YES
NiMH PRESENT
CONTINUE CHARGING
PULSE LED SLOWLY
STOP CHARGING
PAUSE CHARGE TIMER
PULSE LED FAST
BATTERY IN REVERSE
STOP CHARGING
PULSE LED FAST
ZINC-AIR BATTERY PRESENT
STOP CHARGING
PULSE LED FAST
CHARGING COMPLETE
STOP CHARGING
LED ON
ALL THE VALUES LISTED ABOVE ARE TYPICAL.
SEE ELECTRICAL CHARACTERISTICS TABLE FOR MORE INFORMATION
Figure 2. Charge Algorithm
LTC4123
10
4123fa
For more information www.linear.com/LTC4123
operaTion
charge resistance of a Zinc-Air battery is higher than a
NiMH battery and therefore the battery voltage of Zinc-Air
rises significantly. An LED connected to CHRG will blink
fast indicating a battery fault condition.
If the LTC4123 is in UVCL mode at the beginning of the
charge cycle (typically 3 seconds after power is first ap-
plied), it is unable to provide full charge current to perform
Zinc-Air battery detection. In this case, a battery fault will
be indicated at CHRG (blink fast). Adjust the magnetic
coupling between the receive and transmit coils to restart
the charging cycle.
When a battery is inserted in reverse or the die temperature
is above 70˚C or below –5˚C, an LED connected to CHRG
will blink fast. Table 1 summarizes the four different pos-
sible states of the CHRG pin when the charger is active.
Table 1. CHRG Pin Status Summary
CHRG Blink Frequency Charge Status
On (Pull-Down) Charging complete
Blink Slow (0.8Hz) Charging
Blink Fast (6Hz) Fault-No Charging; Temperature Fault/
Battery in Reverse/Zinc-Air Battery
Present/UVCL at the beginning of
charge cycle
Off (High Impedance) No power/No Charging
Operation without Wireless Power
LTC4123 can be powered by connecting a DC voltage
source to the VCC pin instead of receiving power wirelessly
through the ACIN pin. Ground the ACIN pin if an input
supply voltage is connected to VCC.
LTC4123
11
4123fa
For more information www.linear.com/LTC4123
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 at 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 LTC4123 internal diode rectifies the AC voltage
at the ACIN pin.
applicaTions inForMaTion
The power transmission range across the air gap 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 4 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 circulates
through the LC tank formed by CTX and LTX. The current
through LTX is shown in Figure 5.
Figure 3. Wireless Power Transfer System
Figure 4. DC/AC Converter, Transmit/Receive Coils, Tuned Resonant LTC4123 Receiver
LRX LTX
IAC-TX IAC-RX
1:n 4123 F03
AIR GAP
R1
205k
C1
4.7µF
CIN
4.7µF
VIN
5V C2
100µF
LTX
7.5µH
LRX
13µH
CRX
33nF
GND
TRANSMITTER RECEIVER
4123 F04
GND
V+
OUT
LED 1.5V
NiMH
ACIN
LTC4123
BAT
CHRG
GND PROG
VCC
fDRIVE = 244kHz
ICHG = 25mA MAX
fLC_TANK = 315kHz
CTX1
33nF
CTX2
1nF
M1
Si2312CDS
SET
DIV
OE
U1
LTC6990 RPROG
953Ω
+
AIR GAP
(3mm-5mm)
LTC4123
12
4123fa
For more information www.linear.com/LTC4123
applicaTions inForMaTion
Figure 5. Current Through Transmit Coil,
LTX, in Transmitter
Figure 6. Voltage on the Drain and Gate
of NMOS, M1, when fTX_TANK = fDRIVE
Figure 7. Voltage on the Drain and Gate of
NMOS, M1, when fTX_TANK = 1.29 • fDRIVE
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 6 and
Figure 7 illustrate the ZVS condition at different fTX-TANK
frequencies.
fTX−TANK
=
1.29 •fDRIVE
fDRIVE is set by resistor RSET in 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:
V
TX−PEAK =
1.038 •
π
•V
IN
And the peak current through LTX is:
ITX−PEAK =
0.36 •V
IN
fTX
−
TANK •LTX
And the RMS current through LTX is:
ITX-RMS = 0.66 • ITX-PEAK
The LC tank at the receiver, LRX and CRX, is tuned to the
same frequency as the driving frequency of the transmit
LC tank:
f
RX−TANK =
f
DRIVE
where fRX-TANK is given by,
fRX−TANK =
1
2•πLRX •CRX
Note: fDRIVE can be easily adjusted therefore it is best
practice to choose fRX-TANK using minimum component
count (i.e. CRX) then adjusting fDRIVE to match.
The amount of AC current in the transmit coil can be
increased by increasing the supply voltage (VIN), de-
creasing the driving frequency (fDRIVE), or decreasing the
inductance (LTX) of the transmit coil. Since the amount of
power transmitted is proportional to the AC current in the
transmit coil, VIN, fDRIVE and LTX can be varied to adjust
the power delivery to the receive coil.
2µs/DIV
0A
4123 F05
500mA/DIV
2µs/DIV
0V
0V
GATE VOLTAGE
2V/DIV
4123 F06
DRAIN VOLTAGE
5V/DIV
2µs/DIV
0V
DRAIN
VOLTAGE
5V/DIV
GATE
VOLTAGE
2V/DIV
0V
4123 F07
LTC4123
13
4123fa
For more information www.linear.com/LTC4123
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, several parameters
can be used to adjust the transmit power of the transmit-
ter shown in Figure 4. These include the supply voltage,
(VIN), the driving frequency (fDRIVE) and the inductance
of the transmit coil (LTX). Transmit power should be set
as low as possible to receive the desired output power at
worst-case coupling conditions (e.g. maximum transmit
distance with the worst-case misalignment). Increased
transmit power can deliver more power to the LTC4123-
based receiver, but care must be taken not to exceed
the rated current of the transmit coil. Furthermore, the
LTC4123 has the ability to shunt excess received power,
but this will start to increase the temperature of the
LTC4123. Since the LTC4123 die temperature is assumed
to be approximately equal to the battery temperature, it is
important to minimize the die temperature rise to maintain
an accurate battery charge voltage.
Using the rated current of the transmit inductor to set an
upper limit, transmit power should be adjusted downward
until charge current is negatively impacted at worst-case
coupling conditions. Charge current can easily be moni-
tored using the PROG pin voltage.
Once the transmit power level is determined, the transmit
and receive coils should be arranged under best-case cou-
pling conditions with a fully-charged battery or a battery
simulator
. In this scenario, the LTC4123 will shunt excess
power. Measure the LTC4123 temperature using an infrared
sensor or use the negative temperature coefficient of the
battery charge voltage as an indication of temperature.
Charge voltage measured under the best-case coupling
condition should be within ten to fifteen millivolts of the
charge voltage measured under worst-case coupling
conditions (given the same battery current).
Single Transistor Transmitter and LTC4123 Receiver –
Design Example
The example in Figure 4 illustrates the design of the reso-
nant coupled single transistor transmitter and LTC4123
charger. The steps needed to complete the design are
reviewed below.
1. Set the charge current for the LTC4123: In this example,
the charge current required is 25mA:
RPROG =
24V
25mA
=960Ω
Since 960Ω is not a standard 1% value, a 953Ω resis-
tor with a 1% tolerance is selected to obtain a charge
current within 1% of the desired value.
2. 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. 33nF is a standard
value for capacitors, therefore 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 1%) tolerance for CRX. A higher
voltage rating usually corresponds to a higher quality
factor which is preferable. However, the higher the
voltage rating, the larger the package size usually is.
3. Set the driving frequency (fDRIVE) for the Single Tran-
sistor 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 in LTC6990 is grounded.
Select a 205kΩ (standard value) resistor with 1% tol-
applicaTions inForMaTion
LTC4123
14
4123fa
For more information www.linear.com/LTC4123
applicaTions inForMaTion
Table 2. Recommended Components for LTC4123 Receiver
Item Part Description Manufacturer/Part Number
CIN CAP, CHIP, X5R, 4.7µF, ±10%, 10V, 0402 Samsung Electro-Mechanics America Inc. CL05A475KP5NRNC
LRX 13µH, 10mm, Receive Coil Würth 760308101208
CRX CAP, CHIP, C0G, 33nF, ±5%, 50V, 0805 or TDK C2012C0G1H333J125AA
CAP, CHIP, C0G, 33nF, ±1%, 50V, 1206 MURATA GCM3195C1H333FA16D
D1 LED, 630nm, Red, 0603, SMD Rohm Semiconductor SML-311UTT86
RPROG RES, CHIP, 953Ω, ±1%, 1/16W, 0402 VISHAY CRCW0402953RFKED
Table 3. Recommended Components for Single Transistor Transmitter
Item Part Description Manufacturer/Part Number
C1 CAP, CHIP, X5R, 4.7μF, ±20%, 6.3V, 0402 TDK C1005X5R0J475M
C2 CAP, CHIP, X5R, 100μF, ±20%, 6.3V, 1206 MURATA GRM31CR60J107ME39L
LTX 7.5µH, 28mm × 15mm, Transmit Coil Würth 760308103206
CTX1 CAP, CHIP, C0G, 33nF, ±5%, 50V, 0805 TDK C2012C0G1H333J125AA
CTX2 CAP, CHIP, C0G, 1nF, ±5%, 50V, 0603 TDK C1608C0G1H102J080AA
M1 MOSFET, N-CH 20V, 6A, SOT-23-3 Vishay Si2312CDS-T1-GE3
RSET RES, CHIP, 205kΩ, ±1%, 1/16W, 0402 Vishay CRCW0402205KFKED
U1 IC, TimerBlox: Voltage Controlled Silicon Oscillator, 2mm × 3mm DFN Linear Tech. LTC6990IDCB
erance. For more information regarding the LTC6990
oscillator see the data sheet.
4. 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 (CTX) 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 and a 1nF capacitor in parallel to obtain
a value 1% of the calculated CTX. The recommended
rating for CTX capacitors is 50V with 5% (or 1%)
tolerance.
5. Verify if the AC current through the transmit coil is
well within the rated current.
In this example, the supply voltage to the basic tran-
sistor transmitter is 5V. The peak AC current through
the transmit (LTX) coil can be calculated:
I
TX–PEAK =
0.36 •V
IN
fTX–TANK •LTX
=
0.36 •5V
315kHz •7.5µH =0.76A
And ITX-RMS = 0.66 • 0.76 = 0.5A
The rated current for the transmit coil is 1.55A (please
see the Würth 760308103206 data sheet for more
information). The ITX–RMS calculated is well below
the rated current.
Verify the transmit power level chosen does not result
in excessive heating of the LTC4123. Please refer to
the Choosing Transmit Power Level section for more
information.
LTC4123
15
4123fa
For more information www.linear.com/LTC4123
applicaTions inForMaTion
Component Selection for Transmitter and Receiver
To ensure optimum performance from the LTC4123 in
the design example discussed in the previous section, it
is recommended to use the components listed in Table 2
and Table 3 for the receiver and transmitter respectively.
Select receive and transmit coil with good quality factors
to improve the overall power transmission efficiency. Use
ferrite to improve the magnetic coupling between 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 receive and transmit LC tanks.
Component Selection for CHRG Status Indicator
The LED connected at CHRG is powered by a 340µA pull-
down current source. Select a high efficiency LED with low
forward voltage drop. Some recommended components
are shown in Table 4.
Table 4. Recommended LED
Manufacturer/Part Number Part Description
SML-311UTT86 Rohm Semiconductor, LED, 630nm,
RED, 0603, SMD
LTST-C193KRKT-5A Lite-On Inc. LED, RED, SMT, 0603
Stability Considerations
The LTC4123 has three control loops: constant-current
(CC), constant-voltage (CV) and undervoltage current limit
(UVCL). In constant-current mode, the PROG pin is in the
feedback loop. An additional pole is created by the PROG
pin capacitance. Therefore, capacitance on this pin must
be kept to a minimum. With no additional capacitance on
the PROG pin, the LTC4123 charger is stable with program
resistor values as high as 23.7kΩ. However, any additional
capacitance on the PROG pin limits the minimum allowed
charge current.
In UVCL mode, the VCC pin is in the feedback loop. Any
series resistance from the supply to the VCC pin and the
decoupling capacitor at VCC pin will create an additional
pole. The series resistance at the VCC pin is highly variable
and is dependent on the LC tank connected at the ACIN
pin. The LTC4123 is internally compensated to operate
with 1µF to 10µF decoupling capacitor and/or up to 100Ω
to 10kΩ equivalent series resistance from the supply to
the VCC pin.
Zinc-Air Battery Detection
During Zinc-Air battery detection, the full programmed
charge current is applied to the battery for up to 80
(TZn-AIR) seconds after the charger is powered on. The
full programmed charge current is necessary to perform
successful Zinc-Air battery detection.
Upon initial application of input power, if the charger is
unable to provide the programmed charge current, it
signals a fault mode and the LED at CHRG will blink fast.
For instance, the programmed charge current could drop
at the beginning of the charge cycle due to misalignment
between transmit and receive coils. To restart a charge
cycle, it is necessary to remove the receiver from the
transmitter’s magnetic field and try again.
At colder temperatures, if multiple charge cycles are initi-
ated with a fully-charged NiMH battery, it is possible for
the LTC4123 to detect that battery as a Zinc-Air battery
and signal a fault (blink fast). This is because the internal
impedance of a fully-charged NiMH battery is significantly
higher at colder temperatures.
Board Layout Considerations
The VCC bypass capacitor should be connected as close
as possible to the VCC pin. The trace connection from
the ground return of the bypass capacitor to the ground
return of the LC tank should be as short as possible to
minimize and localize AC noise. To minimize the parasitic
capacitance on the PROG pin, the trace connection from
the PROG pin to the programming resistor should be
as short as possible. The ground return for the resistor
should be connected to GND via the exposed pad with the
shortest possible trace length.
LTC4123
16
4123fa
For more information www.linear.com/LTC4123
package DescripTion
Please refer to http://www.linear.com/product/LTC4123#packaging for the most recent package drawings.
2.00 ±0.10
(4 SIDES)
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WCCD-2)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
0.40 ±0.10
BOTTOM VIEW—EXPOSED PAD
0.60 ±0.10
(2 SIDES)
0.75 ±0.05
R = 0.125
TYP
R = 0.05
TYP
1.37 ±0.10
(2 SIDES)
1
3
64
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DC6) DFN REV C 0915
0.25 ±0.05
0.50 BSC
0.25 ±0.05
1.37 ±0.10
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.60 ±0.10
(2 SIDES)
1.15 ±0.05
0.70 ±0.05
2.55 ±0.05
PACKAGE
OUTLINE
0.50 BSC
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
DC6 Package
6-Lead Plastic DFN (2mm × 2mm)
(Reference LTC DWG # 05-08-1703 Rev C)
LTC4123
17
4123fa
For more information www.linear.com/LTC4123
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 07/16 Modified Charge Voltage Limit in characteristics table.
Modified CHRG pin description.
Modified Block Diagram of CHRG pin. Corrected polarity symbol of comparator in Block Diagram.
Modified Input Voltage Qualification section.
Modified Table 2 and Table 3.
Modified Component Selection for CHRG Status Indicator section.
3
6
7
8
14
15
LTC4123
18
4123fa
For more information www.linear.com/LTC4123
LINEAR TECHNOLOGY CORPORATION 2015
LT 0716 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4123
relaTeD parTs
Typical applicaTion
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.
LTC4125 5W AutoResonant Wireless Power
Transmitter
Monolithic AutoResonant Full Bridge Driver. Transmit power automatically adjusts to
receiver load, Foreign Object Detection, Wide Operating Switching Frequency Range:
50kHz-250kHz, Input Voltage Range 3V to 5.5V, 20-Lead 4mm × 5mm QFN Package
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.
Wireless 25mA p675 NiMH Linear Charger Tuned at 244kHz
Wireless 25mA p675 NiMH Linear Charger Tuned at 255kHz
R1
205k
C1
4.7µF
CIN
4.7µF
VIN
5V C2
100µF
LTX
7.5µH
LRX
13µH
CRX
33nF
GND
4123 TA03
GND
V+
OUT
LED
1.5V
POWER ONE
NiMH
(P675)
ACIN
LTC4123
BAT
CHRG
GND PROG
VCC
fDRIVE = 244kHz
ICHG = 25mA MAX
fLC_TANK = 315kHz
CTX1
33nF
CTX2
1nF
CTX1, CRX: C2012C0G1H333J125AA
CTX2: C1608C0G1H102J080AA
LTX: 760308103206
LRX: 760308101208
M1
Si2312CDS
SET
DIV
OE
U1
LTC6990 RPROG
953Ω
+
AIR GAP
(3mm-5mm)
R1
196k
C1
4.7µF
CIN
4.7µF
VIN
5V C2
100µF
LTX
5.9µH
LRX
5.8µH
CRX
68nF
GND
4123 TA04
GND
V+
OUT
LED
1.5V
POWER ONE
NiMH
(P675)
ACIN
LTC4123
BAT
CHRG
GND PROG
VCC
fDRIVE = 255kHz
ICHG = 25mA MAX
fLC_TANK = 329kHz
CTX1
33nF
CTX2
6.8nF
CTX1: C2012C0G1H333J125AA
CTX2: C1608C0G1H682J080AA
LTX: L41200T23
CRX: GRM31C5C1H683JA01L
LRX: L4120R19
M1
Si2312CDS
SET
DIV
OE
U1
LTC6990 RPROG
953Ω
+
AIR GAP
(4mm – 6.5mm)