19-1426; Rev 0; 2/99
NUAL
KIT MA
ATION
EET
H
S
A
EVALU
T
WS DA
FOLLO
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
The MAX1687/MAX1688 step-up DC-DC converters deliver 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 slowly charging a reservoir capacitor, which supplies the necessary 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% conversion efficiency and eliminates the need for an external
Schottky diode. A logic shutdown mode reduces the shutdown 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, controlled 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 constantrecharge-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
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
Ordering Information
TEMP. RANGE
PIN-PACKAGE
MAX1687EUE
-40°C to +85°C
16 TSSOP
MAX1687ESA
MAX1688EUE
MAX1688ESA
*Ug.
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
8 SO
16 TSSOP
8 SO
PART*
PC Cards (PCMCIA)
Typical Operating Circuit
Pin Configurations
2.7V TO 6V
TOP VIEW
IN
IN 1
16 OUT
IN 2
15 OUT
LX1 3
14 LX2
LX1 4
LIM [CHG] 5
MAX1687
MAX1688
1 Li-lon
OR
3 NiMH
OR
3 ALKALINE
LX2
OUT
MAX1687
MAX1688
13 LX2
ON
12 PGND
FB 6
11 PGND
REF 7
10 AGND
N.C. 8
9
OFF
0 TO 1V
CONTROL INPUT
VOUT
UP TO 6V
FB
ON
(LIM)
[CHG]
REF
ON
TSSOP
Pin Configurations continued at end of data sheet.
GND
[ ] ARE FOR MAX1688
†Pg
LX1
( ) ARE FOR MAX1687
[ ] ARE FOR MAX1688
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX1687 /MAX1688 †
General Description
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
ABSOLUTE MAXIMUM RATINGS
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
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.
ELECTRICAL CHARACTERISTICS
(VIN = VON = +3V, VLIM = 1V (MAX1687), VCHG = 1V (MAX1688), VFB = 1.5V, VOUT = 6V, TA = 0°C to +85°C, unless otherwise noted.
Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
Input Voltage Range
IN rising, 1% hysteresis
Output Voltage Range
VREF
gmFB
ICHG Source Current
Peak Current
IPEAK
IRIPPLE
Sense Resistor
RSENSE
ON Input High Voltage
Input Current
VIL
VIH
UNITS
6
V
2.6
V
6
V
2
4
mA
Shutdown, VIN = 4.2V, LX2 connected to LX1,
VOUT = 0, ON = GND
3
10
µA
0.7
1.2
1.8
ms
IREF = 0 to 10µA
1.225
1.25
1.275
V
VFB rising, 2% hysteresis
1.212
1.250
1.288
V
VFB = 1.125V, VOUT = 3V (MAX1688)
0.18
0.2
0.22
mmho
60
110
VFB = 0, VOUT = 3V (MAX1688)
Ripple Current
ON Input Low Voltage
2.5
MAX
VFB = 1.5V
tDELAY
FB Set Voltage
FB Transconductance
2.4
VREF
Input Supply Current
Reference Voltage
TYP
2.7
Input Undervoltage Lockout
Shutdown Delay
MIN
µA
VLIM = VCHG = 1V
0.744
0.8
0.856
VLIM = VCHG = 0.65V
0.46
0.5
0.54
VLIM = VCHG = 1V
170
200
230
mA
0.1
0.18
Ω
0.6
V
VIN = 2.7V
VIN = 6V
1.8
VIN = 4.2V
1.5
V
IFB
VFB = 1.5V
0.05
0.2
ION
VON = 0 or 3V
0.02
0.1
0.02
0.1
ILIM
VLIM = 1V
TA = +25°C
A
TA = 0°C to +85°C
µA
2
N-Channel On-Resistance
VIN = 2.7V
0.4
0.8
Ω
P-Channel On-Resistance
VIN = 2.7V
0.3
0.7
Ω
Precharge On-Resistance
VIN = 4V, VFB = 0, VOUT = 0
30
70
Ω
LX2 Leakage Current
VIN = VLX2 = 6V, VOUT = VON = 0
0.05
10
µA
2
_______________________________________________________________________________________
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
(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)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
2.7
6
V
2.35
2.65
V
VREF
6
V
VFB = 1.5V
5
mA
Shutdown VIN = 4.2V, LX2 connected to LX1,
VOUT = 0, ON = GND
10
µA
Input Voltage Range
Input Undervoltage Lockout
IN rising, 1% hysteresis
Output Voltage Range
IIN
Input Supply Current
ISHDN
Shutdown Delay
0.6
2
ms
IREF = 0 to 10µA
1.212
1.288
V
FB rising, 2% hysteresis
1.20
1.30
V
VFB = 1.125V, VOUT = 3V (MAX1688)
0.16
0.24
mmho
VLIM = VCHG = 1V
0.73
0.90
VLIM = VCHG = 0.65V
0.44
0.57
VLIM = VCHG = 1V
145
240
mA
0.18
Ω
0.6
V
TDELAY
Reference Voltage
VREF
FB Set Voltage
FB Transconductance
gmFB
Peak Current
IPEAK
Ripple Current
IRIPPLE
Sense Resistor
RSENSE
ON Input Low Voltage
VIL
ON Input High Voltage
TYP
VIN = 2.7V
VIH
VIN = 6V
1.8
VIN = 4.2V
1.5
A
V
N-Channel On-Resistance
VIN = 2.7V
0.8
Ω
P-Channel On-Resistance
VIN = 2.7V
0.7
Ω
Precharge On-Resistance
VIN = 4V, VFB = 0, VOUT = 0
70
Ω
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25°C, unless otherwise noted.)
EFFICIENCY vs. GSM BURST LOAD
(VOUT = 5.5V)
90
VIN = 5V
90
75
VIN = 2.7V
65
80
VIN = 3.3V
75
VIN = 2.7V
70
VIN = 6V
100
150
200
250
LOAD CURRENT (mA)
300
350
85
80
75
70
65
60
50
90
VIN = 6V
65
60
0
95
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 3.3V
70
VIN = 5V
85
85
80
100
MAX1687/88 toc02
95
MAX1687/88 toc01
95
EFFICIENCY vs. LOAD CURRENT
(VIN = 2.7V, VOUT = 3.3V)
MAX1687/88 toc03
EFFICIENCY vs. DC LOAD CURRENT
(VOUT = 5.5V)
60
0
500
1000 1500 2000
LOAD CURRENT (mA)
2500
3000
0
50
100
150
200
250
300
350
LOAD CURRENT (mA)
_______________________________________________________________________________________
3
MAX1687/MAX1688
ELECTRICAL CHARACTERISTICS
_____________________________Typical Operating Characteristics (continued)
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25°C, unless otherwise noted.)
MAX1688 PEAK BATTERY CURRENT
vs. RCHG (1A GSM LOAD)
400
300
3.0
1.210
1.205
VIN = 2.7V
20
25
30
35
40
1.200
-40
RCHG (kΩ)
-20
0
20
40
60
80
1
100
10
1.249
MAX1687/88 toc08
1200
1000
FREQUENCY (kHz)
1.251
1000
SWITCHING FREQUENCY vs. INDUCTANCE
(VIN = 3.3V, VOUT = 5V,
ILOAD = 100mA, VLIM = 1V)
MAX1687/88 toc07
1.253
100
IREF (µA)
TEMPERATURE (°C)
REFERENCE VOLTAGE vs. TEMPERATURE
(VIN = 3.3V, VOUT = 5V)
REFERENCE VOLTAGE (V)
1.225
1.215
VIN = 3.3V
1.5
15
1.230
1.220
VIN = 5V
2.5
2.0
0
MAX1687/88 toc06
1.235
3.5
200
100
1.245
1.240
VREF (V)
500
VIN = 6V
4.0
1.250
MAX1687/88 toc05
600
4.5
SUPPLY CURRENT (mA)
700
REFERENCE VOLTAGE vs. REFERENCE
CURRENT (VIN = 3.3V, VOUT = 5V)
NO-LOAD BATTERY INPUT CURRENT vs.
TEMPERATURE (VOUT = 5V, VLIM = 1V )
MAX1687/88 toc04
800
PEAK BATTERY CURRENT (mA)
800
600
400
1.247
200
0
1.245
0
20
40
60
80
100
10
100
INDUCTANCE (µH)
MAX1688
IPEAK vs. VOUT DROOP
MAX1688 PEAK INDUCTOR CURRENT
vs. RCHG (1A GSM LOAD)
MAX1687/88 toc09
800
750
700
650
600
550
500
450
400
RCHG = 40.2k
350
800
700
600
500
400
300
200
100
0
300
100
150
200
250
300
VOUT DROOP (mV)
4
1
TEMPERATURE (°C)
MAX1687/88 toc10
-20
PEAK INDUCTOR CURRENT (mA)
-40
IPEAK (mA)
MAX1687/MAX1688
Step-Up DC-DC Converters with Precise,
Adaptive Current Limit for GSM
350
400
15
20
25
30
RCHG (kΩ)
_______________________________________________________________________________________
35
40
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25°C, unless otherwise noted.)
MAX1688
SWITCHING WAVEFORMS
(GSM PULSED LOAD 1A, RCHG = 40.2kΩ)
SWITCHING WAVEFORMS
(FIXED ILOAD = 300mA)
MAX1687/88 toc10a
MAX1687/88 toc11
500mA/div
ILX
500mA/div
ILOAD
ILX
500mA/div
VOUT
VOUT
200mV/div
100mV/div
1ms/div
500µs/div
RCHL = 40.2kΩ, L = 10µH
MAX1688
SWITCHING WAVEFORMS
(GSM PULSED LOAD 1A, RCHG = 18kΩ)
INDUCTOR CURRENT
MAX1687/88 toc12
MAX1687/88 toc13
500mA/div
ILX
500mA/div
ILOAD
ILX
VOUT
200mV/div
VLIM = 1V
ILX
200mA/div
VLIM = 0
0A
1ms/div
2µs/div
RCHG = 18kΩ, L = 10µH
POWER-UP WAVEFORM
(RLOAD = 15Ω COUT = 2000µF)
VON vs. BATTERY CURRENT
MAX1687/88 toc14
MAX1687/88 toc15
VON
VOUT
2V/div
VON
1V/div
IBATTERY
200mA/div
1V/div
5ms/div
10µs/div
_______________________________________________________________________________________
5
MAX1687/MAX1688
Typical Operating Characteristics (continued)
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
MAX1687/MAX1688
Pin Description
PIN
MAX1687
MAX1688
NAME
FUNCTION
Supply Voltage Input. Connect Battery to IN. Bypass to GND with
a 47µF minimum capacitor.
SO
TSSOP
SO
TSSOP
1
1, 2
1
1, 2
IN
2
3, 4
2
3, 4
LX1
Internal Current-Sense Resistor Output. Connect the inductor
between LX1 and LX2.
3
5
—
—
LIM
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.
—
—
3
5
CHG
Constant-Recharge-Time Input. Set the recharge time of the output reservoir capacitor by connecting a resistor from CHG to GND
(see Applications Information section).
4
6
4
6
FB
—
7
—
7
REF
Reference Voltage Output. 1.25V nominal.
—
8
—
8
N.C.
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.
5
9
5
9
ON
6
—
6
—
GND
—
10
—
10
AGND
Analog Ground
—
11, 12
—
11, 12
PGND
Power Ground
7
13, 14
7
13, 14
LX2
N-Channel and P-Channel MOSFET Drain
8
15, 16
8
15, 16
OUT
Output
Detailed Description
The MAX1687 and MAX1688 ICs supply power amplifiers 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 volt6
Feedback Input. Connect a resistor-divider from OUT to GND to
set the output voltage. FB regulates to a nominal 1.25V.
Ground
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 circuitry that depends on the battery to malfunction or be
reset. The MAX1687/MAX1688 prevent battery voltage
droop by charging the reservoir capacitor during system off-time and isolate the battery from the output during 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
MAX1687/MAX1688
LX2
LX1
VOUT
Q1
VIN
P-SWITCH
Q2
P-SWITCH
ZERO
CROSSING
N-SWITCH
CONSTANT
HYSTERETIC
INDUCTOR-CURRENT
CONTROL LOGIC
Q3
PEAK/
TROUGH
INDUCTORCURRENT
DETECT
gm
REF
FB
(LIM)
[CHG]
MAX1687
MAX1688
gm
TIMER
VPRECHARGE
VIN
VIN - VDIODE
VOUT
ON
( ) ARE FOR MAX1687
[ ] ARE FOR MAX1688 (ALSO DASHED LINES)
Figure 1. Functional Diagram
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 30Ω precharge 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 transition 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 inductor current oscillates between I PEAK - 200mA and
IPEAK.
Standby/Shutdown
When ON goes low, the device enters Standby Mode,
inductor current ramps to zero, and the output disconnects from the input. If ON remains low for greater than
1.2ms (typ), the device shuts down and quiescent current drops to 3µA (typ).
_______________________________________________________________________________________
7
CURRENT
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
ILOAD
IPEAK
SET BY
VLIM (VCHG)
HYSTERESIS
BAND
IPEAK - 200mA
VOUT
“ON”
CONTROL INPUT
TIME
TIME
( ) ARE FOR MAX1688
Figure 2. Hysteretic Inductor Current
Figure 4. Timing Diagram of “ON”
Applications Information
Adjusting the Output Voltage
VOUT
OUT
Adjust the MAX1687/MAX1688 output voltage with two
external resistors (Figure 3). Choose R2 to be between
10kΩ to 100kΩ. Calculate R1 as follows:
R1
MAX1687
MAX1688
R1 = R2 · (VOUT - VFB ) / VFB
FB
where VFB is the feedback threshold voltage, 1.25V
nominal.
R2
Adjusting Current Limit (MAX1687)
R1 = R2
V
-V
( OUTVFB FB )
Figure 3. Setting the Output Voltage
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).
The MAX1687 has an adjustable current limit for applications 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-current 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.
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 voltages).
8
_______________________________________________________________________________________
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
MAX1687
REF
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:
b)
MAX1687
LIM
DAC
c)
LIM
(
REF
R3
MAX1687
LIM
VLIM(CHG) = VREF
R4
R4 + R3
IBURST ⋅ VOUT ⋅ DGSM
RCHG =
VIN(MIN) ⋅ 1 - DGSM
VDROOP
R3 + R4 > 125kΩ
R4
(
)
) + 0.1 ⋅
VIN(MIN)
⋅
gmCHG
⋅
VREF
⋅
gmFB
⋅
1 - tol
(
)
where:
Figure 5. Current-Limit Adjust
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 voltage 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 current. This results in a constant recharge time that compensates for varying output filter capacitor characteristics 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
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 current 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 40.2kΩ RCHG is shorter than
that with an 18kΩ RCHG, 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 frequency. Calculate the switching frequency as:
f = VIN [1 - (VIN / VOUT)] / (L · IRIPPLE)
where f is the switching frequency, VIN is the input voltage, 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 frequency 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).
_______________________________________________________________________________________
9
MAX1687/MAX1688
a)
MAX1687/MAX1688
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
Output (Reservoir) Capacitor
Typical Application Circuits
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:
The current limit of the MAX1687 can be set by an external DAC (Figure 6a), making it variable by using a microcontroller. 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.
C
OUT
=
( DROOP
V
D
GSM
- I
BURST
⋅
⋅
I
BURST
ESR
⋅
t
GSM
OUTPUT CAPACITOR
)(1 - tol)
Layout
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 tolerance, and VDROOP is the acceptable drop in the output 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 ripple is:
VRIPPLE = IRIPPLE · ESR(OUTPUT CAPACITOR)
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 lowinductance 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.
where IRIPPLE is typically 0.2A.
10µH
VIN
2.7V TO 6V
LX1
LX2
LX1
LX2
IN
47µF
0.1µF
MAX1687
IN
VOUT = 5V
2A AT 12%
DUTY CYCLE
OUT
OUT
R2
187k
DAC
OUTPUT
0 TO 1V
LIM
ON
ON
FB
AGND
OFF
PGND
REF
2000µF
R1
61.9k
PGND
Figure 6a. MAX1687 Typical Application Circuit (GSM Pulsed Load)
10
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Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
MAX1687/MAX1688
10µH
VIN
2.7V TO 6V
LX1
LX2
LX1
LX2
IN
MAX1687
OUT
IN
0.1µF
47µF
R2
187k
REF*
ON
VOUT = 5V
350mA
OUT
47µF
FB
AGND
ON
OFF
PGND
R1
61.9k
PGND
LIM
*TSSOP PACKAGE ONLY
Figure 6b. MAX1687 Typical Application Circuit (Fixed Non-Pulsed Load)
10µH
VIN
2.7V TO 6V
LX1
LX2
LX1
LX2
IN
MAX1688
IN
0.1µF
OUT
47µF
R2
187k
CHG
RCHG
40.2k
ON
VOUT = 5V
2A AT 12%
DUTY CYCLE
OUT
ON
2000µF
FB
AGND
OFF
PGND
REF
R1
61.9k
PGND
Figure 7. MAX1688 Typical Application Circuit (GSM Pulsed Load)
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11
Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM
MAX1687/MAX1688
Pin Configurations (continued)
Table 1. Component Suppliers
COMPANY
TOP VIEW
IN 1
8
LX1 2
LIM [CHG]
3
MAX1687
MAX1688
FB 4
OUT
7
LX2
6
GND
5
ON
FAX
PHONE
AVX
207-283-1941
207-282-5111
CoilCraft
708-639-6400
708-639-1469
Coiltronics
561-241-9339
561-241-7876
404-736-3030
404-736-1300
81-3-3607-5428
708-956-0666
Murata-Erie
Sumida
Chip Information
TRANSISTOR COUNT: 1920
SO
[ ] ARE FOR MAX1688
TSSOP.EPS
Package Information
12
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