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19-1426; Rev 0; 2/99
NUAL KIT MA ATION ET EVALU TA SHE WS DA FOLLO
Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM
General Description Features
o Low 450mA Peak Battery Current Provides 2A, 5V GSM Burst o 90% Efficiency o Internal Power MOSFETs and Current-Sense Resistor o Output Disconnects from Input During Shutdown o 3A Shutdown Current o Precise Voltage-Controlled Current Limit (MAX1687) o Adaptive Constant-Recharge-Time Capability (MAX1688) o 1.25V to 6V Adjustable Output o 2.7V to 6V Input Range (1 Li-Ion cell or 3 NiMH cells) o Switching Frequency Can Exceed 1MHz o Standby Mode Disables DC-DC During Transmission Burst o Low Inrush Current at Start-Up
MAX1687 /MAX1688
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 3A. 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.
Ordering Information
PART* MAX1687EUE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 TSSOP 8 SO 16 TSSOP 8 SO
Applications
GSM Phones Wireless Handsets PC Cards (PCMCIA)
-40C to +85C MAX1687ESA MAX1688EUE -40C to +85C MAX1688ESA -40C to +85C *U.S. and foreign patents pending.
Typical Operating Circuit
2.7V TO 6V IN LX1 LX2 OUT VOUT UP TO 6V
Pin Configurations
TOP VIEW
IN 1 IN 2 LX1 3 LX1 4 LIM [CHG] 5 FB 6 REF 7 N.C. 8 [ ] ARE FOR MAX1688 16 OUT 15 OUT 14 LX2
1 Li-lon OR 3 NiMH OR 3 ALKALINE ON
MAX1687 MAX1688
ON (LIM) REF GND
MAX1687 MAX1688
13 LX2 12 PGND 11 PGND 10 AGND 9 ON
( ) ARE FOR MAX1687 [ ] ARE FOR MAX1688 OFF 0 TO 1V CONTROL INPUT
FB
[CHG]
TSSOP Pin Configurations continued at end of data sheet.
Patent pending
________________________________________________________________ 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.
Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
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 = +70C) TSSOP (derate 5.7mW/C above +70C) ....................457mW SO (derate 5.88mW/C above +70C) .........................471mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C 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 = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Input Voltage Range Input Undervoltage Lockout Output Voltage Range VFB = 1.5V Input Supply Current Shutdown Delay Reference Voltage FB Set Voltage FB Transconductance ICHG Source Current Peak Current Ripple Current Sense Resistor ON Input Low Voltage ON Input High Voltage IPEAK IRIPPLE RSENSE VIL VIH IFB Input Current ION ILIM N-Channel On-Resistance P-Channel On-Resistance Precharge On-Resistance LX2 Leakage Current VIN = 2.7V VIN = 6V VIN = 4.2V VFB = 1.5V VON = 0 or 3V VLIM = 1V VIN = 2.7V VIN = 2.7V VIN = 4V, VFB = 0, VOUT = 0 VIN = VLX2 = 6V, VOUT = VON = 0 TA = +25C TA = 0C to +85C 0.4 0.3 30 0.05 1.8 1.5 0.05 0.02 0.02 0.2 0.1 0.1 2 0.8 0.7 70 10 A A gmFB tDELAY VREF IREF = 0 to 10A VFB rising, 2% hysteresis VFB = 1.125V, VOUT = 3V (MAX1688) VFB = 0, VOUT = 3V (MAX1688) VLIM = VCHG = 1V VLIM = VCHG = 0.65V VLIM = VCHG = 1V Shutdown, VIN = 4.2V, LX2 connected to LX1, VOUT = 0, ON = GND 0.7 1.225 1.212 0.18 60 0.744 0.46 170 IN rising, 1% hysteresis SYMBOL CONDITIONS MIN 2.7 2.4 VREF 2 3 1.2 1.25 1.250 0.2 110 0.8 0.5 200 0.1 0.856 0.54 230 0.18 0.6 2.5 TYP MAX 6 2.6 6 4 10 1.8 1.275 1.288 0.22 UNITS V V V mA A ms V V mmho A A mA V V
2
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM
ELECTRICAL CHARACTERISTICS
(VIN = VON = +3V, VLIM = 1V (MAX1687), VCHG = 1V (MAX1688), VFB = 1.5V, VOUT = 6V, TA = -40C to +85C, unless otherwise noted.) (Note 1) PARAMETER Input Voltage Range Input Undervoltage Lockout Output Voltage Range IIN Input Supply Current Shutdown Delay Reference Voltage FB Set Voltage FB Transconductance Peak Current Ripple Current Sense Resistor ON Input Low Voltage ON Input High Voltage N-Channel On-Resistance P-Channel On-Resistance Precharge On-Resistance gmFB IPEAK IRIPPLE RSENSE VIL VIH VIN = 2.7V VIN = 6V VIN = 4.2V VIN = 2.7V VIN = 2.7V VIN = 4V, VFB = 0, VOUT = 0 1.8 1.5 0.8 0.7 70 ISHDN TDELAY VREF IREF = 0 to 10A FB rising, 2% hysteresis VFB = 1.125V, VOUT = 3V (MAX1688) VLIM = VCHG = 1V VLIM = VCHG = 0.65V VLIM = VCHG = 1V VFB = 1.5V Shutdown VIN = 4.2V, LX2 connected to LX1, VOUT = 0, ON = GND 0.6 1.212 1.20 0.16 0.73 0.44 145 IN rising, 1% hysteresis SYMBOL CONDITIONS MIN 2.7 2.35 VREF TYP MAX 6 2.65 6 5 10 2 1.288 1.30 0.24 0.90 0.57 240 0.18 0.6 UNITS V V V mA A ms V V mmho A mA V V
MAX1687/MAX1688
Note 1: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. DC LOAD CURRENT (VOUT = 5.5V)
MAX1687/88 toc01
EFFICIENCY vs. GSM BURST LOAD (VOUT = 5.5V)
MAX1687/88 toc02
EFFICIENCY vs. LOAD CURRENT (VIN = 2.7V, VOUT = 3.3V)
95 90 EFFICIENCY (%) 85 80 75 70
MAX1687/88 toc03
95 90 VIN = 5V 85 EFFICIENCY (%)
95 90 85 EFFICIENCY (%) 80 75 VIN = 2.7V 70 65 60 VIN = 6V VIN = 3.3V VIN = 5V
100
80 75 70 65 60 0 50
VIN = 3.3V VIN = 2.7V VIN = 6V
65 60 0 500 1000 1500 2000 LOAD CURRENT (mA) 2500 3000 0 50 100 150 200 250 300 350 LOAD CURRENT (mA)
100
150
200
250
300
350
LOAD CURRENT (mA)
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
_____________________________Typical Operating Characteristics (continued)
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25C, unless otherwise noted.)
MAX1688 PEAK BATTERY CURRENT vs. RCHG (1A GSM LOAD)
MAX1687/88 toc04
NO-LOAD BATTERY INPUT CURRENT vs. TEMPERATURE (VOUT = 5V, VLIM = 1V )
MAX1687/88 toc05
REFERENCE VOLTAGE vs. REFERENCE CURRENT (VIN = 3.3V, VOUT = 5V)
1.245 1.240 1.235 VREF (V) 1.230 1.225 1.220 1.215
MAX1687/88 toc06
800 700 PEAK BATTERY CURRENT (mA) 600 500 400 300 200 100 0 15 20 25 30 35
4.5 4.0 SUPPLY CURRENT (mA) 3.5 3.0 2.5 2.0 1.5 VIN = 5V VIN = 3.3V VIN = 2.7V VIN = 6V
1.250
1.210 1.205 1.200 80 100 1 10 IREF (A) 100 1000
40
-40
-20
0
20
40
60
RCHG (k)
TEMPERATURE (C)
REFERENCE VOLTAGE vs. TEMPERATURE (VIN = 3.3V, VOUT = 5V)
MAX1687/88 toc07
SWITCHING FREQUENCY vs. INDUCTANCE (VIN = 3.3V, VOUT = 5V, ILOAD = 100mA, VLIM = 1V)
MAX1687/88 toc08
1.253
1200 1000 FREQUENCY (kHz) 800 600 400 200
REFERENCE VOLTAGE (V)
1.251
1.249
1.247
1.245 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0 1 10 INDUCTANCE (H) 100
MAX1688 IPEAK vs. VOUT DROOP
750 700 650 IPEAK (mA) 600 550 500 450 400 350 300 100 150 200 250 300 350 400 VOUT DROOP (mV) RCHG = 40.2k
MAX1687/88 toc09
MAX1688 PEAK INDUCTOR CURRENT vs. RCHG (1A GSM LOAD)
700 PEAK INDUCTOR CURRENT (mA) 600 500 400 300 200 100 0 15 20 25 30 35 40 RCHG (k)
MAX1687/88 toc10
800
800
4
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM
Typical Operating Characteristics (continued)
(VIN = +3.3V, VOUT = 5V, VLIM = 1V, Figures 6b and 7, TA = +25C, unless otherwise noted.)
MAX1688 SWITCHING WAVEFORMS (GSM PULSED LOAD 1A, RCHG = 40.2k)
MAX1687/88 toc11
MAX1687/MAX1688
SWITCHING WAVEFORMS (FIXED ILOAD = 300mA)
MAX1687/88 toc10a
500mA/div 500mA/div ILX
ILX
500mA/div
ILOAD
VOUT 100mV/div 200mV/div
VOUT
500s/div
1ms/div RCHL = 40.2k, L = 10H
MAX1688 SWITCHING WAVEFORMS (GSM PULSED LOAD 1A, RCHG = 18k)
MAX1687/88 toc12
INDUCTOR CURRENT
MAX1687/88 toc13
500mA/div
ILX
ILX 500mA/div ILOAD VLIM = 1V
VOUT 200mV/div
200mA/div 0A
ILX VLIM = 0 2s/div
1ms/div RCHG = 18k, L = 10H
POWER-UP WAVEFORM (RLOAD = 15 COUT = 2000F)
MAX1687/88 toc14
VON vs. BATTERY CURRENT
MAX1687/88 toc15
VON 2V/div VOUT VON 1V/div
IBATTERY 200mA/div 1V/div
5ms/div
10s/div
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
Pin Description
PIN MAX1687 SO TSSOP 1, 2 3, 4 MAX1688 SO 1 2 TSSOP 1, 2 3, 4 IN LX1 Supply Voltage Input. Connect Battery to IN. Bypass to GND with a 47F minimum capacitor. Internal Current-Sense Resistor Output. Connect the inductor between LX1 and LX2. 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. 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). 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 3A in shutdown mode. Ground Analog Ground Power Ground N-Channel and P-Channel MOSFET Drain Output NAME FUNCTION
1
2
3
5
--
--
LIM
--
--
3
5
CHG
4 -- --
6 7 8
4 -- --
6 7 8
FB REF N.C.
5
9
5
9
ON
6 -- -- 7 8
-- 10 11, 12 13, 14 15, 16
6 -- -- 7 8
-- 10 11, 12 13, 14 15, 16
GND AGND PGND LX2 OUT
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
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.
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
LX1 LX2 VOUT P-SWITCH Q2 P-SWITCH ZERO CROSSING
VIN
Q1
N-SWITCH CONSTANT HYSTERETIC INDUCTOR-CURRENT CONTROL LOGIC
Q3
gm
PEAK/ TROUGH INDUCTORCURRENT DETECT
REF
FB (LIM) [CHG] gm
MAX1687 MAX1688
TIMER
VPRECHARGE
VIN VOUT
VIN - VDIODE
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 30 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 3A (typ).
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
CURRENT
IPEAK SET BY VLIM (VCHG) HYSTERESIS BAND IPEAK - 200mA
ILOAD
VOUT
"ON" CONTROL INPUT
TIME ( ) ARE FOR MAX1688
TIME
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 10k to 100k. Calculate R1 as follows: R1 = R2 * (VOUT - VFB ) / VFB
MAX1687 MAX1688
FB
R1
where VFB is the feedback threshold voltage, 1.25V nominal.
R2
Adjusting Current Limit (MAX1687)
V -V
R1 = R2
( OUTFB FB ) V
Synchronized ON Pin
Figure 3. Setting the Output Voltage
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).
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM
a) b)
MAX1687
REF LIM DAC LIM
MAX1687
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:
IBURST VOUT DGSM RCHG = VIN(MIN) 1 - DGSM
MAX1687/MAX1688
c)
R3
(
REF VLIM(CHG) = VREF R3 + R4 > 125k R4 R4 + R3
(
)
) + 0.1

MAX1687
LIM
R4
VDROOP
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 10A. 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 = 200A/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 = 31.5k, using a 5% tolerance resistor. The recovery time for a 40.2k RCHG is shorter than that with an 18k RCHG, but the peak battery current is higher. See Switching Waveforms (GSM Pulsed Load 1A, RCHG = 40.2k) and Switching Waveforms (GSM Pulsed Load 1A, RCH = 18k) 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).
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
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:
C =
Typical Application Circuits
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.
OUT
( DROOP
V
D -I
GSM
I
BURST
BURST
t
GSM
ESR
OUTPUT CAPACITOR
)(1 - tol)
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 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 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) where IRIPPLE is typically 0.2A.
10H
LX1 LX1 VIN 2.7V TO 6V 0.1F 47F IN IN
LX2 LX2 VOUT = 5V 2A AT 12% DUTY CYCLE
MAX1687
OUT OUT R2 187k
2000F
DAC OUTPUT 0 TO 1V ON OFF
LIM
FB
ON
AGND PGND R1 61.9k
REF
PGND
Figure 6a. MAX1687 Typical Application Circuit (GSM Pulsed Load)
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
10H
LX1 LX1 VIN 2.7V TO 6V 0.1F 47F IN IN
LX2 LX2
MAX1687
OUT OUT R2 187k
VOUT = 5V 350mA
47F
REF*
FB
ON OFF
ON
AGND PGND R1 61.9k
LIM
PGND
*TSSOP PACKAGE ONLY
Figure 6b. MAX1687 Typical Application Circuit (Fixed Non-Pulsed Load)
10H
LX1 LX1 VIN 2.7V TO 6V 0.1F 47F IN IN
LX2 LX2 VOUT = 5V 2A AT 12% DUTY CYCLE R2 187k
MAX1688
OUT OUT
2000F
CHG
FB
RCHG 40.2k
ON OFF
ON
AGND PGND R1 61.9k
REF
PGND
Figure 7. MAX1688 Typical Application Circuit (GSM Pulsed Load)
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Step-Up DC-DC Converters with Precise, Adaptive Current Limit for GSM MAX1687/MAX1688
Pin Configurations (continued)
TOP VIEW
Table 1. Component Suppliers
COMPANY AVX CoilCraft Coiltronics FAX 207-283-1941 708-639-6400 561-241-9339 404-736-3030 81-3-3607-5428 PHONE 207-282-5111 708-639-1469 561-241-7876 404-736-1300 708-956-0666
IN 1 LX1 2 LIM [CHG] 3
8 7
OUT LX2 GND ON
Murata-Erie Sumida
MAX1687 MAX1688
6 5
Chip Information
TRANSISTOR COUNT: 1920
FB 4
SO
[ ] ARE FOR MAX1688
Package Information
TSSOP.EPS
12
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