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General Description
The MAX1687/MAX1688 step-up DC-DC converters deliv-
er up to 2W from a single Li-Ion or three NiMH cells. The
devices are ideal for burst-load applications such as GSM
cell phones and wireless LANs, where the RF power
amplifiers require short, high current bursts. The
MAX1687/MAX1688 reduce battery surge current by slow-
ly charging a reservoir capacitor, which supplies the nec-
essary peak energy for the load current burst. As a result,
the peak battery current is limited, thus maximizing battery
life and minimizing battery voltage sag and transient dips.
An internal synchronous rectifier provides over 90% con-
version efficiency and eliminates the need for an external
Schottky diode. A logic shutdown mode reduces the shut-
down current to only 3µA. The devices can be disabled
during current bursts (RF transmit mode) to eliminate
switching noise.
The switching frequency of the MAX1687/MAX1688, con-
trolled by the selected inductor, can exceed 1MHz. Two
external resistors set the output voltage from 1.25V to 6V.
The MAX1687 controls peak battery current, while the
MAX1688 features a more advanced, adaptive constant-
recharge-time algorithm that maximizes battery life. The
MAX1687/MAX1688 are available in thin 16-pin TSSOP
(1.1mm max height) or standard 8-pin SO packages.
Applications
GSM Phones
Wireless Handsets
PC Cards (PCMCIA)
Features
♦Low 450mA Peak Battery Current
Provides 2A, 5V GSM Burst
♦90% Efficiency
♦Internal Power MOSFETs and Current-Sense
Resistor
♦Output Disconnects from Input During Shutdown
♦3µA Shutdown Current
♦Precise Voltage-Controlled Current Limit
(MAX1687)
♦Adaptive Constant-Recharge-Time Capability
(MAX1688)
♦1.25V to 6V Adjustable Output
♦2.7V to 6V Input Range
(1 Li-Ion cell or 3 NiMH cells)
♦Switching Frequency Can Exceed 1MHz
♦Standby Mode Disables DC-DC During
Transmission Burst
♦Low Inrush Current at Start-Up
MAX1687/MAX1688
†
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
________________________________________________________________
Maxim Integrated Products
1
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
IN OUT
OUT
LX2
LX2
PGND
PGND
AGND
ON
TOP VIEW
MAX1687
MAX1688
TSSOP
IN
LX1
FB
LX1
LIM [CHG]
REF
N.C.
[ ] ARE FOR MAX1688
MAX1687
MAX1688
VOUT
UP TO 6V
OUT
IN LX1 LX2
ON
ON
2.7V TO 6V
OFF 0 TO 1V
CONTROL INPUT
1 Li-lon
OR
3 NiMH
OR
3 ALKALINE
(LIM)
REF
FB
[CHG]
( ) ARE FOR MAX1687
[ ] ARE FOR MAX1688
GND
Typical Operating Circuit
19-1426; Rev 0; 2/99
PART*
MAX1687EUE
MAX1687ESA
MAX1688EUE -40°C to +85°C
-40°C to +85°C
-40°C to +85°C
TEMP. RANGE PIN-PACKAGE
16 TSSOP
8 SO
16 TSSOP
Pin Configurations
Ordering Information
MAX1688ESA -40°C to +85°C 8 SO
Pin Configurations continued at end of data sheet.
†
Pg
*
Ug.
EVALUATION KIT MANUAL
FOLLOWS DATA SHEET
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = VON = +3V, VLIM = 1V (MAX1687), VCHG = 1V (MAX1688), VFB = 1.5V, V OUT = 6V, TA = 0°C to +85°C , unless otherwise noted.
Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
IN, ON, LX1, CHG, LIM, FB, OUT, REF to GND.......-0.3V to +7V
LX2 to GND ..............................................................-0.3V to +8V
IN, LX1 Average Current..........................................................1A
Continuous Power Dissipation (TA= +70°C)
TSSOP (derate 5.7mW/°C above +70°C) ....................457mW
SO (derate 5.88mW/°C above +70°C).........................471mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10sec)............................+300° C
IN rising, 1% hysteresis
VFB = 0, VOUT = 3V (MAX1688)
VFB = 1.125V, VOUT = 3V (MAX1688)
VFB rising, 2% hysteresis
IREF = 0 to 10µA
VFB = 1.5V
Shutdown, VIN = 4.2V, LX2 connected to LX1,
VOUT = 0, ON = GND
VLIM = VCHG = 1V
CONDITIONS
0.744 0.8 0.856
Peak Current
VVREF 6Output Voltage Range V
2.4 2.5 2.6 V2.7 6Input Voltage Range
Input Undervoltage Lockout
µA60 110ICHG Source Current mmho0.18 0.2 0.22gmFB
FB Transconductance V1.212 1.250 1.288FB Set Voltage V1.225 1.25 1.275VREF
Reference Voltage
mA
24
Input Supply Current µA310
UNITSMIN TYP MAXSYMBOLPARAMETER
VLIM = VCHG = 1V
VLIM = VCHG = 0.65V
VIN = 2.7V
VLIM = 1V
VON = 0 or 3V
VIN = 2.7V
VIN = 6V
VIN = 4.2V
VFB = 1.5V
VIN = 2.7V Ω0.3 0.7P-Channel On-Resistance
Ω0.1 0.18RSENSE
Sense Resistor mA170 200 230IRIPPLE
A
0.46 0.5 0.54
IPEAK
Ripple Current
Ω0.4 0.8N-Channel On-Resistance 2
0.02 0.1
ILIM
0.02 0.1ION
V0.6VIL
V
1.8
ON Input Low Voltage
1.5 0.05 0.2IFB
Input Current
VIN = VLX2 = 6V, VOUT = VON = 0
VIN = 4V, VFB = 0, VOUT = 0 µA0.05 10 Ω30 70Precharge On-Resistance
LX2 Leakage Current
ms0.7 1.2 1.8tDELAY
Shutdown Delay
VIH
ON Input High Voltage
µA
TA= 0°C to +85°C
TA= +25°C
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS
(VIN = VON = +3V, VLIM = 1V (MAX1687), VCHG = 1V (MAX1688), VFB = 1.5V, VOUT = 6V, TA= -40°C to +85°C, unless otherwise noted.)
(Note 1)
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
VLIM = VCHG = 1V
VLIM = VCHG = 0.65V
IIN
IN rising, 1% hysteresis
VFB = 1.125V, VOUT = 3V (MAX1688)
FB rising, 2% hysteresis
IREF = 0 to 10µA
VFB = 1.5V
Shutdown VIN = 4.2V, LX2 connected to LX1,
VOUT = 0, ON = GND
VLIM = VCHG = 1V
CONDITIONS
mA
145 240
IRIPPLE
Ripple Current
A
0.44 0.57
IPEAK
Ω
0.18
RSENSE
Sense Resistor
0.73 0.90
Peak Current
V
2.35 2.65 V
2.7 6
Input Voltage Range
Input Undervoltage Lockout
mmho
0.16 0.24
gmFB
FB Transconductance V
1.20 1.30
FB Set Voltage V1.212 1.288VREF
Reference Voltage ms
0.6 2
TDELAY
Shutdown Delay
mA
5
ISHDN
Input Supply Current µA10
UNITSMIN TYP MAXSYMBOLPARAMETER
VIN = 4V, VFB = 0, VOUT = 0
VIN = 2.7V
VIN = 2.7V
Ω
70
Precharge On-Resistance Ω
0.7 Ω
0.8
N-Channel On-Resistance
P-Channel On-Resistance
Typical Operating Characteristics
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA= +25°C, unless otherwise noted.)
60
70
65
80
75
85
95
90
0 150 20050 100 250 300 350
EFFICIENCY vs. DC LOAD CURRENT
(VOUT = 5.5V)
MAX1687/88 toc01
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 5V
VIN = 3.3V
VIN = 2.7V
VIN = 6V
60
65
70
75
80
85
90
95
0 1500500 1000 2000 2500 3000
EFFICIENCY vs. GSM BURST LOAD
(VOUT = 5.5V)
MAX1687/88 toc02
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 5V
VIN = 3.3V
VIN = 2.7V
VIN = 6V
60
70
65
80
75
95
90
85
100
0 10050 150 200 250 300 350
EFFICIENCY vs. LOAD CURRENT
(VIN = 2.7V, VOUT = 3.3V)
MAX1687/88 toc03
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 2.7V V
0.6
VIL
ON Input Low Voltage
V
VIN = 6V
VIN = 4.2V
VIH
ON Input High Voltage 1.8
1.5
V
VREF 6
Output Voltage Range
MAX1687/MAX1688
Step-Up DC-DC Converters with Precise,
Adaptive Current Limit for GSM
4 _______________________________________________________________________________________
_____________________________
Typical Operating Characteristics (continued)
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA= +25°C, unless otherwise noted.)
300
450
400
350
500
550
600
650
700
750
800
100 200150 250 300 350 400
MAX1688
IPEAK vs. VOUT DROOP
MAX1687/88 toc09
VOUT DROOP (mV)
IPEAK (mA)
RCHG = 40.2k
0
800
700
600
500
400
300
200
100
15 20 25 30 35 40
MAX1688 PEAK INDUCTOR CURRENT
vs. RCHG (1A GSM LOAD)
MAX1687/88 toc10
RCHG (kΩ)
PEAK INDUCTOR CURRENT (mA)
0
100
200
300
400
500
600
700
800
15 20 25 30 35 40
MAX1688 PEAK BATTERY CURRENT
vs. RCHG (1A GSM LOAD)
MAX1687/88 toc04
RCHG (kΩ)
PEAK BATTERY CURRENT (mA)
1.5
2.5
2.0
3.5
3.0
4.0
4.5
-40 20 40-20 0 60 80 100
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
NO-LOAD BATTERY INPUT CURRENT vs.
TEMPERATURE (VOUT = 5V, VLIM = 1V )
MAX1687/88 toc05
VIN = 5V
VIN = 3.3V
VIN = 2.7V
VIN = 6V
REFERENCE VOLTAGE vs. REFERENCE
CURRENT (VIN = 3.3V, VOUT = 5V)
MAX1687/88 toc06
IREF (µA)
VREF (V)
1.245
1.200 1 100010010
1.215
1.205
1.235
1.225
1.250
1.220
1.210
1.240
1.230
1200
01 10 100
SWITCHING FREQUENCY vs. INDUCTANCE
(VIN = 3.3V, VOUT = 5V,
ILOAD = 100mA, VLIM = 1V)
200
MAX1687/88 toc08
INDUCTANCE (µH)
FREQUENCY (kHz)
600
400
1000
800
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
_______________________________________________________________________________________
5
Typical Operating Characteristics (continued)
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA= +25°C, unless otherwise noted.)
500mA/div
500mA/div
200mV/div VOUT
1ms/div
ILX
ILOAD
MAX1688
SWITCHING WAVEFORMS
(GSM PULSED LOAD 1A, RCHG = 40.2kΩ)
MAX1687/88 toc11
RCHL = 40.2kΩ, L = 10µH
200mA/div
0A
VLIM = 1V
VLIM = 0
2µs/div
ILX
ILX
INDUCTOR CURRENT
MAX1687/88 toc13
200mA/div
1V/div
IBATTERY
VON
10µs/div
MAX1687/88 toc15
VON vs. BATTERY CURRENT
2V/div
1V/div
VON
VOUT
5ms/div
MAX1687/88 toc14
POWER-UP WAVEFORM
(RLOAD = 15Ω COUT = 2000µF)
500mA/div
500mA/div
200mV/div VOUT
1ms/div
ILX
ILOAD
MAX1688
SWITCHING WAVEFORMS
(GSM PULSED LOAD 1A, RCHG = 18kΩ)
MAX1687/88 toc12
RCHG = 18kΩ, L = 10µH
500mA/div
100mV/div VOUT
500µs/div
ILX
SWITCHING WAVEFORMS
(FIXED ILOAD = 300mA)
MAX1687/88 toc10a
MAX1687/MAX1688
Detailed Description
The MAX1687 and MAX1688 ICs supply power ampli-
fiers in GSM applications where limited input current
surge is desirable. For example, GSM systems require
high-power, 12% duty-cycle RF bursts. Synchronizing
the MAX1687/MAX1688 to enter standby mode during
these RF bursts eliminates battery surge current and
minimizes switching noise to the power amplifier. In
standby mode, the charged output reservoir capacitor
delivers power to the power amplifier. Between each
burst, the DC-DC converter switches on to charge the
output capacitor. To improve efficiency and reduce peak
battery current, the MAX1687/MAX1688 provide a volt-
age-controlled current limit. The MAX1688 is a MAX1687
with added self-regulating circuitry that recharges the
reservoir capacitor in a fixed time (Figure 1).
Start-Up Sequence
In a conventional DC-DC converter, when high current
is required by the load, the battery voltage droops due
to battery series resistance. This may cause other cir-
cuitry that depends on the battery to malfunction or be
reset. The MAX1687/MAX1688 prevent battery voltage
droop by charging the reservoir capacitor during sys-
tem off-time and isolate the battery from the output dur-
ing high current demand. The MAX1687/MAX1688 are
gentle to the battery during initial power-up, as well.
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
6 _______________________________________________________________________________________
Pin Description
PIN
NAME FUNCTION
MAX1688MAX1687
—
10
11, 12
13, 14
15, 16
6
7
8
9
5
—
3, 4
1, 2
TSSOP
GND
AGND
PGND
LX2
OUT
FB
REF
N.C.
ON
CHG
LIM
LX1
IN
Ground
Analog Ground
Power Ground
N-Channel and P-Channel MOSFET Drain
Output
Feedback Input. Connect a resistor-divider from OUT to GND to
set the output voltage. FB regulates to a nominal 1.25V.
Reference Voltage Output. 1.25V nominal.
No Connection. Not internally connected.
Logic ON/OFF Input. When ON is high, the device operates in
normal mode. When ON goes low, the device goes into standby
mode. If ON remains low for greater than 1.2ms, the device shuts
down (see
Standby/Shutdown
section). The supply current falls to
3µA in shutdown mode.
Constant-Recharge-Time Input. Set the recharge time of the out-
put reservoir capacitor by connecting a resistor from CHG to GND
(see
Applications Information
section).
Voltage-Controlled Current-Limit Adjust Input. Apply a voltage
between 0 and 1V to vary the current limit. LIM is internally
clamped to 1.25V.
Internal Current-Sense Resistor Output. Connect the inductor
between LX1 and LX2.
Supply Voltage Input. Connect Battery to IN. Bypass to GND with
a 47µF minimum capacitor.
6—6
—10—
—11, 12—
713, 147
815, 168
464
—7—
—8—
595
3——
—53
SO
23, 42
11, 2
1
SOTSSOP
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
_______________________________________________________________________________________ 7
When starting up, the MAX1687/MAX1688 employ four
successive phases of operation to reduce the inrush of
current from the battery. These phases are Linear
Regulator Mode, Pseudo Buck Mode, Pseudo Boost
Mode, and Boost Mode. In Linear Mode, the output
connects to the input through a 30Ωprecharge PMOS
device (Figure1, Q1). The transition from Linear Mode
to Pseudo Buck Mode occurs when VOUT = VIN - 3V.
The transition from Pseudo Buck Mode to Pseudo
Boost Mode occurs when VOUT = VIN - 0.7V. The tran-
sition from Pseudo Boost Mode to Boost Mode occurs
when VOUT > VIN. Due to these mode changes, the
battery input current remains relatively constant, and
VOUT changes slope as it rises.
Hysteretic Inductor-Current Control
Logic circuits in the MAX1687/MAX1688 control the
inductor ripple current to typically 200mA (Figure 2).
The voltage at LIM (CHG) programs IPEAK. The induc-
tor current oscillates between IPEAK - 200mA and
IPEAK.
Standby/Shutdown
When ON goes low, the device enters Standby Mode,
inductor current ramps to zero, and the output discon-
nects from the input. If ON remains low for greater than
1.2ms (typ), the device shuts down and quiescent cur-
rent drops to 3µA (typ).
Q3
REF
FB
VOUT
VOUT
VPRECHARGE VIN
gm
ON
( ) ARE FOR MAX1687
[ ] ARE FOR MAX1688 (ALSO DASHED LINES)
(LIM)
[CHG]
gm
VIN - VDIODE
CONSTANT
HYSTERETIC
INDUCTOR-CURRENT
CONTROL LOGIC
PEAK/
TROUGH
INDUCTOR-
CURRENT
DETECT
Q2
P-SWITCH
N-SWITCH
ZERO
CROSSING
P-SWITCH
LX2
Q1
LX1
VIN
MAX1687
MAX1688
TIMER
Figure 1. Functional Diagram
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
8 _______________________________________________________________________________________
Synchronized ON Pin
If desired, drive ON low during periods of high current
demand to eliminate switching noise from affecting
sensitive RF circuitry. During the periods when ON is
low, the output reservoir capacitor provides current to
the load (Figure 4).
Buck Capability
Although the IC is not intended for this application, the
MAX1687/MAX1688 operate as a buck converter when
the input voltage is higher than the output voltage. The
MAX1687/MAX1688 are not optimally efficient in this
mode (see
Typical Operating Characteristics
for
efficiencies at 2.7V, 3.3V, 5V, and 6V input supply volt-
ages).
Applications Information
Adjusting the Output Voltage
Adjust the MAX1687/MAX1688 output voltage with two
external resistors (Figure 3). Choose R2 to be between
10kΩto 100kΩ. Calculate R1 as follows:
R1 = R2 · (VOUT - VFB ) / VFB
where VFB is the feedback threshold voltage, 1.25V
nominal.
Adjusting Current Limit (MAX1687)
The MAX1687 has an adjustable current limit for appli-
cations requiring limited supply current, such as PC
card sockets or applications with variable burst loads.
For single Li-Ion battery cell applications, the high peak
current demands of the RF transmitter power amplifier
can pull the battery very low as the battery impedance
increases toward the end of discharge. The reservoir
capacitor at the output supplies power during load-cur-
rent bursts; this allows for a lower input current limit.
With this feature, the life of the Li-Ion battery versus the
reservoir capacitor size trade-off can be optimized for
each application.
( ) ARE FOR MAX1688
IPEAK - 200mA
IPEAK
SET BY
VLIM (VCHG)HYSTERESIS
BAND
CURRENT
TIME
Figure 2. Hysteretic Inductor Current
OUT VOUT
R1
R2
R1 = R2 VOUT - VFB
VFB
FB
( )
MAX1687
MAX1688
Figure 3. Setting the Output Voltage
“ON”
CONTROL INPUT
VOUT
ILOAD
TIME
Figure 4. Timing Diagram of “ON”
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
_______________________________________________________________________________________ 9
To set the current limit, apply a voltage of 0 to 1V at
LIM. The current limit is 200mA when VLIM = 0 to
0.25V. Use the following equation to calculate ILIM:
ILIM = VLIM (0.86A/V) – 0.06A
where VLIM = 0.25V to 1V.
VLIM is internally clamped to 1.25V when the voltage
applied at VLIM is above 1.25V. Generate VLIM by one
of three methods: an externally applied voltage, the
output of a DAC, or a resistor-divider using VREF as the
supply voltage (TSSOP packages) (Figure 5). Note that
REF can supply up to 10µA.
Determine VLIM as follows:
VLIM = (ILX(PEAK) + 0.06A) / 0.86
where ILX(PEAK) = [(ILOAD · VOUT) / VIN ] + 0.1A (see
the Inductor Current parameter in the
Typical Operating
Characteristics
).
Setting Recharge Time (MAX1688)
The MAX1688 has a recharging feature employing a
sample-and-hold, which sets the maximum time to
recharge the reservoir capacitor. Synchronize the ON
pin to place the converter in standby during each load
current burst. At the end of each load current burst, the
output voltage is sampled by the MAX1688. This volt-
age controls the peak inductor current. The greater the
difference between the regulated output voltage and
the valley of the sag voltage, the higher the peak cur-
rent. This results in a constant recharge time that com-
pensates for varying output filter capacitor character-
istics as well as a varying input voltage. Therefore, the
circuit demands only as much peak current from the
battery as output conditions require, minimizing the
peak current from the battery. An external resistor
between CHG and GND controls the output recharge
time. A large resistor increases peak inductor current
which speeds up recovery time. Calculate the resistor
as follows:
where:
RCHG is the external resistor
IBURST is the peak burst current expected
DGSM is the duty cycle of GSM
VIN is the input voltage
VOUT is the output voltage
VREF = 1.25V
VDROOP is the drop in output voltage during the cur-
rent burst
gmCHG is the internal transconductance = 0.8A/V
gmFB is the feedback transconductance = 200µA/V
tol is the tolerance of the RCHG resistor
For example, for IBURST = 2.66A, VDROOP = 0.36V, VIN
= +2.7V, and VOUT = 3.6V, then RCHG = 31.5kΩ, using
a 5% tolerance resistor.
The recovery time for a 40.2kΩRCHG is shorter than
that with an 18kΩRCHG, but the peak battery current is
higher. See Switching Waveforms (GSM Pulsed Load
1A, RCHG = 40.2kΩ) and Switching Waveforms (GSM
Pulsed Load 1A, RCH = 18kΩ) in
Typical Operating
Characteristics
.
Inductor Selection
The value of the inductor determines the switching fre-
quency. Calculate the switching frequency as:
f = VIN [1 - (VIN / VOUT)] / (L · IRIPPLE)
where f is the switching frequency, VIN is the input volt-
age, VOUT is the output voltage, L is the inductor value,
and IRIPPLE is the ripple current expected, typically
0.2A. Using a lower value inductor increases the fre-
quency and reduces the physical size of the inductor.
A typical frequency is from 150kHz to 1MHz (see
Switching Frequency vs. Inductance in the
Typical
Operating Characteristics
).
DAC REF
REF
a)
c)
b)
LIM
LIM
VLIM(CHG) = VREF
R3 + R4 > 125kΩ
R4
R4 + R3
R3
R4
LIM
MAX1687
MAX1687 MAX1687
Figure 5. Current-Limit Adjust
R = I V D
V 1 - D + 0.1
V
V gm V gm 1 - tol
CHG BURST OUT GSM
IN(MIN) GSM
IN(MIN)
DROOP CHG REF FB
⋅⋅
⋅⋅
⋅⋅⋅⋅
()
()
()
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
10 ______________________________________________________________________________________
Output (Reservoir) Capacitor
The value of the output capacitor determines the
amount of power available to deliver to the power
amplifier during the RF burst. A larger output capacitor
with low ESR reduces the amount of output voltage
droop during an RF burst. Use the following equation to
determine capacitor size when ON is synchronized to
the RF burst:
where COUT is the output capacitor, IBURST is the peak
power amplifier burst current, tGSM is the current pulse
period, DGSM is the duty cycle, tol is the capacitor tol-
erance, and VDROOP is the acceptable drop in the out-
put during the current burst.
For example, when used in a typical GSM system,
tGSM = 4.62ms, IBURST = 2.66A for a +3.6V system
(1.42A for a +5.5V system), and with a droop of less
than 10%, the value of the capacitor is 5.3mF ±20%.
The output capacitor also determines the constant-load
(ON connected to VCC) ripple voltage. The output rip-
ple is:VRIPPLE = IRIPPLE · ESR(OUTPUT CAPACITOR)
where IRIPPLE is typically 0.2A.
Typical Application Circuits
The current limit of the MAX1687 can be set by an exter-
nal DAC (Figure 6a), making it variable by using a micro-
controller. The MAX1687 is the choice for systems
interfacing with a microcontroller, but may also be used
with fixed current limit (Figure 6b). The MAX1688 can
monitor the droop of the output voltage to set the current
limit, maximizing battery life. The MAX1688 is suitable for
systems demanding variable burst currents (Figures 6a,
6b, and 7) as well as variable input voltages.
Layout
The MAX1687/MAX1688’s high-frequency operation
and high peak currents make PC board layout critical
to minimize ground bounce and noise. Locate input
bypass and output filter capacitors as close to the
device pins as possible. All connections to OUT and FB
should also be kept as short as possible. Use a low-
inductance ground plane. Connect the ground leads of
the input capacitor, output capacitor, and PGND pins in
a star configuration to the ground plane. Table 1 lists
suggested suppliers. Refer to the MAX1687/MAX1688
evaluation kit manual for a suggested surface-mount
layout and a list of suggested components.
C = D I t
V - I ESR 1 - tol
OUT GSM BURST GSM
DROOP BURST OUTPUT CAPACITOR
⋅⋅
⋅
()()
VOUT = 5V
2A AT 12%
DUTY CYCLE
OUT
OUT
IN
IN
0.1µF47µF
VIN
2.7V TO 6V
10µH
2000µF
R2
187k
R1
61.9k
LX1
LX1
LX2
LX2
ON
ON
OFF
DAC
OUTPUT
0 TO 1V LIM
REF
FB
AGND
PGND
PGND
MAX1687
Figure 6a. MAX1687 Typical Application Circuit (GSM Pulsed Load)
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
______________________________________________________________________________________________________ 11
OUT
OUT
IN
IN
0.1µF47µF
VIN
2.7V TO 6V
RCHG
40.2k
10µH
2000µF
R2
187k
R1
61.9k
LX1
LX1
LX2
LX2
ON
ON
OFF
REF
CHG FB
AGND
PGND
PGND
VOUT = 5V
2A AT 12%
DUTY CYCLE
MAX1688
Figure 7. MAX1688 Typical Application Circuit (GSM Pulsed Load)
VOUT = 5V
350mA
OUT
OUT
IN
IN
0.1µF47µF
VIN
2.7V TO 6V
10µH
47µF
R2
187k
R1
61.9k
LX1
LX1
LX2
LX2
ON
ON
OFF
LIM
REF* FB
AGND
PGND
PGND
*TSSOP PACKAGE ONLY
MAX1687
Figure 6b. MAX1687 Typical Application Circuit (Fixed Non-Pulsed Load)
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
12 ______________________________________________________________________________________
Package Information
Table 1. Component Suppliers
COMPANY FAX PHONE
AVX 207-283-1941 207-282-5111
CoilCraft 708-639-6400 708-639-1469
Coiltronics 561-241-9339 561-241-7876
Murata-Erie 404-736-3030 404-736-1300
Sumida 81-3-3607-5428 708-956-0666
Chip Information
TRANSISTOR COUNT: 1920
Pin Configurations (continued)
GND
ONFB
1
2
8
7
OUT
LX2LX1
LIM [CHG]
IN
SO
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