Datasheet MAX1688EUE, MAX1688ESA, MAX1687ESA Datasheet (Maxim)

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General Description

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
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

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.

†

Patent pending

*

U.S. and foreign patents pending.

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

2.7V TO 6V

1 Li-lon

OR

3 NiMH

OR

3 ALKALINE

ON

OFF

0 TO 1V

CONTROL INPUT

( ) ARE FOR MAX1687
[ ] ARE FOR MAX1688

LX1 LX2

IN

MAX1687
MAX1688

ON
(LIM)

REF

GND

OUT

FB

[CHG]

V

OUT

UP TO 6V

MAX1687/MAX1688

Step-Up DC-DC Converters with
Precise, Adaptive Current Limit for GSM

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VIN= VON= +3V, V

LIM

= 1V (MAX1687), V

CHG

= 1V (MAX1688), VFB= 1.5V, V

OUT

= 6V, TA = 0°C to +85°C, unless otherwise noted.

Typical values are at T

A

= +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, V

OUT

= 3V (MAX1688)

VFB= 1.125V, V

OUT

= 3V (MAX1688)

VFBrising, 2% hysteresis

I

REF

= 0 to 10µA

VFB= 1.5V
Shutdown, VIN= 4.2V, LX2 connected to LX1,

V

OUT

= 0, ON = GND

V

LIM

= V

CHG

= 1V

CONDITIONS

0.744 0.8 0.856

Peak Current

VV

REF

6Output Voltage Range

V

2.4 2.5 2.6

V2.7 6Input Voltage Range

Input Undervoltage Lockout

µA60 110I

CHG

Source Current

mmho0.18 0.2 0.22g

mFB

FB Transconductance

V1.212 1.250 1.288FB Set Voltage

V1.225 1.25 1.275V

REF

Reference Voltage

mA

24

Input Supply Current

µA310

UNITSMIN TYP MAXSYMBOLPARAMETER

V

LIM

= V

CHG

= 1V

V

LIM

= V

CHG

= 0.65V

VIN= 2.7V

V

LIM

= 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 0.18R

SENSE

Sense Resistor

mA170 200 230I

RIPPLE

A

0.46 0.5 0.54

I

PEAK

Ripple Current

Ω0.4 0.8N-Channel On-Resistance

2

0.02 0.1

I

LIM

0.02 0.1I

ON

V0.6V

IL

V

1.8

ON Input Low Voltage

1.5

0.05 0.2I

FB

Input Current

VIN= V

LX2

= 6V, V

OUT

= VON= 0

VIN= 4V, V

FB

= 0, V

OUT

= 0

µA0.05 10

Ω30 70Precharge On-Resistance

LX2 Leakage Current

ms0.7 1.2 1.8t

DELAY

Shutdown Delay

V

IH

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, V

LIM

= 1V (MAX1687), V

CHG

= 1V (MAX1688), VFB= 1.5V, V

OUT

= 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.

V

LIM

= V

CHG

= 1V

V

LIM

= V

CHG

= 0.65V

I

IN

IN rising, 1% hysteresis

VFB= 1.125V, V

OUT

= 3V (MAX1688)

FB rising, 2% hysteresis

I

REF

= 0 to 10µA

VFB= 1.5V
Shutdown VIN= 4.2V, LX2 connected to LX1,

V

OUT

= 0, ON = GND

V

LIM

= V

CHG

= 1V

CONDITIONS

mA

145 240

I

RIPPLE

Ripple Current

A

0.44 0.57

I

PEAK

Ω

0.18

R

SENSE

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

gm

FB

FB Transconductance

V

1.20 1.30

FB Set Voltage

V1.212 1.288V

REF

Reference Voltage

ms

0.6 2

T

DELAY

Shutdown Delay

mA

5

I

SHDN

Input Supply Current

µA10

UNITSMIN TYP MAXSYMBOLPARAMETER

VIN= 4V, V

FB

= 0, V

OUT

= 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, V

OUT

= 5V, V

LIM

= 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

(V

OUT

= 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

(V

OUT

= 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

(V

IN

= 2.7V, V

OUT

= 3.3V)

MAX1687/88 toc03

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN= 2.7V V

0.6

V

IL

ON Input Low Voltage

V

VIN= 6V
VIN= 4.2V

V

IH

ON Input High Voltage

1.8

1.5

V

V

REF

6

Output Voltage Range

MAX1687/MAX1688

Step-Up DC-DC Converters with Precise,
Adaptive Current Limit for GSM

4 _______________________________________________________________________________________

_____________________________

Typical Operating Characteristics (continued)

(V

IN

= +3.3V, V

OUT

= 5V, V

LIM

= 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

I

PEAK

vs. V

OUT

DROOP

MAX1687/88 toc09

V

OUT

DROOP (mV)

I

PEAK

(mA)

R

CHG

= 40.2k

0

800
700
600
500
400
300
200

100

15 20 25 30 35 40

MAX1688 PEAK INDUCTOR CURRENT

vs. R

CHG

(1A GSM LOAD)

MAX1687/88 toc10

R

CHG

(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. R

CHG

(1A GSM LOAD)

MAX1687/88 toc04

R

CHG

(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 (V

OUT

= 5V, V

LIM

= 1V )

MAX1687/88 toc05

VIN = 5V

VIN = 3.3V

VIN = 2.7V

VIN = 6V

REFERENCE VOLTAGE vs. REFERENCE

CURRENT (V

IN

= 3.3V, V

OUT

= 5V)

MAX1687/88 toc06

I

REF

(µA)

V

REF

(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

1.245

1.247

1.249

1.251

1.253

TEMPERATURE (°C)

REFERENCE VOLTAGE (V)

-40 20 40-20 0 60 80 100

REFERENCE VOLTAGE vs. TEMPERATURE

(V

IN

= 3.3V, V

OUT

= 5V)

MAX1687/88 toc07

1200

0

1 10 100

SWITCHING FREQUENCY vs. INDUCTANCE

(V

IN

= 3.3V, V

OUT

= 5V,

I

LOAD

= 100mA, V

LIM

= 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)

(V

IN

= +3.3V, V

OUT

= 5V, V

LIM

= 1V, Figures 6b and 7, TA= +25°C, unless otherwise noted.)

500mA/div

500mA/div

200mV/div

V

OUT

1ms/div

I

LX

I

LOAD

MAX1688

SWITCHING WAVEFORMS

(GSM PULSED LOAD 1A, R

CHG

= 40.2kΩ)

MAX1687/88 toc11

R

CHL

= 40.2kΩ, L = 10µH

200mA/div

0A

V

LIM = 1V

V

LIM

= 0

2µs/div

I

LX

I

LX

INDUCTOR CURRENT

MAX1687/88 toc13

200mA/div

1V/div

I

BATTERY

V

ON

10µs/div

MAX1687/88 toc15

VON vs. BATTERY CURRENT

2V/div

1V/div

V

ON

V

OUT

5ms/div

MAX1687/88 toc14

POWER-UP WAVEFORM

(R

LOAD

= 15Ω C

OUT

= 2000µF)

500mA/div

500mA/div

200mV/div

V

OUT

1ms/div

I

LX

I

LOAD

MAX1688

SWITCHING WAVEFORMS

(GSM PULSED LOAD 1A, R

CHG

= 18kΩ)

MAX1687/88 toc12

R

CHG

= 18kΩ, L = 10µH

500mA/div

100mV/div

V

OUT

500µs/div

I

LX

SWITCHING WAVEFORMS

(FIXED I

LOAD

= 300mA)

MAX1687/88 toc10a

MAX1687/MAX1688

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 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 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

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 out­put 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 30Ω precharge PMOS
device (Figure1, Q1). The transition from Linear Mode
to Pseudo Buck Mode occurs when V

OUT

= VIN- 3V.
The transition from Pseudo Buck Mode to Pseudo
Boost Mode occurs when V

OUT

= VIN- 0.7V. The tran­sition from Pseudo Boost Mode to Boost Mode occurs
when V

OUT

> VIN. Due to these mode changes, the
battery input current remains relatively constant, and
V

OUT

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 I

PEAK

. The induc-

tor current oscillates between I

PEAK

- 200mA and

I

PEAK

.

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).

Q3

REF

FB

V

OUT

V

OUT

V

PRECHARGE

V

IN

g

m

ON

( ) ARE FOR MAX1687
[ ] ARE FOR MAX1688 (ALSO DASHED LINES)

(LIM)

[CHG]

g

m

VIN - V

DIODE

CONSTANT

HYSTERETIC

INDUCTOR-CURRENT

CONTROL LOGIC

PEAK/

TROUGH

INDUCTOR-

CURRENT

DETECT

Q2
P-SWITCH

N-SWITCH

ZERO
CROSSING

P-SWITCH

LX2

Q1

LX1

V

IN

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 volt­ages).

Applications Information

Adjusting the Output Voltage

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 = R2 · (V

OUT

- VFB) / V

FB

where VFBis the feedback threshold voltage, 1.25V
nominal.

Adjusting Current Limit (MAX1687)

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.

( ) ARE FOR MAX1688

I

PEAK

- 200mA

I

PEAK

SET BY

V

LIM (VCHG

)

HYSTERESIS
BAND

CURRENT

TIME

Figure 2. Hysteretic Inductor Current

OUT

V

OUT

R1

R2

R1 = R2

V

OUT

- V

FB

V

FB

FB

( )

MAX1687
MAX1688

Figure 3. Setting the Output Voltage

“ON”

CONTROL INPUT

V

OUT

I

LOAD

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 V

LIM

= 0 to

0.25V. Use the following equation to calculate I

LIM

:

I

LIM

= V

LIM

(0.86A/V) – 0.06A

where V

LIM

= 0.25V to 1V.

V

LIM

is internally clamped to 1.25V when the voltage

applied at V

LIM

is above 1.25V. Generate V

LIM

by one
of three methods: an externally applied voltage, the
output of a DAC, or a resistor-divider using V

REF

as the
supply voltage (TSSOP packages) (Figure 5). Note that
REF can supply up to 10µA.

Determine V

LIM

as follows:

V

LIM

= (I

LX(PEAK)

+ 0.06A) / 0.86

where I

LX(PEAK)

= [(I

LOAD

· V

OUT

) / 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

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:

R

CHG

is the external resistor

I

BURST

is the peak burst current expected

D

GSM

is the duty cycle of GSM
VINis the input voltage
V

OUT

is the output voltage

V

REF

= 1.25V

V

DROOP

is the drop in output voltage during the cur-

rent burst
g

mCHG

is the internal transconductance = 0.8A/V

g

mFB

is the feedback transconductance = 200µA/V

tol is the tolerance of the R

CHG

resistor

For example, for I

BURST

= 2.66A, V

DROOP

= 0.36V, V

IN

= +2.7V, and V

OUT

= 3.6V, then R

CHG

= 31.5kΩ, using

a 5% tolerance resistor.
The recovery time for a 40.2kΩ R

CHG

is shorter than

that with an 18kΩ R

CHG

, but the peak battery current is
higher. See Switching Waveforms (GSM Pulsed Load
1A, R

CHG

= 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/ V

OUT

)] / (L · I

RIPPLE

)

where f is the switching frequency, VINis the input volt­age, V

OUT

is the output voltage, L is the inductor value,

and I

RIPPLE

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

).

DAC REF

REF

a)

c)

b)

LIM

LIM

V

LIM(CHG)

= V

REF

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 C

OUT

is the output capacitor, I

BURST

is the peak

power amplifier burst current, t

GSM

is the current pulse

period, D

GSM

is the duty cycle, tol is the capacitor tol-

erance, and V

DROOP

is the acceptable drop in the out-

put during the current burst.
For example, when used in a typical GSM system,

t

GSM

= 4.62ms, I

BURST

= 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:

V

RIPPLE

= I

RIPPLE

· ESR

(OUTPUT CAPACITOR)

where I

RIPPLE

is typically 0.2A.

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.

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.

C =

D I t

V - I ESR 1 - tol

OUT

GSM BURST GSM

DROOP BURST OUTPUT CAPACITOR

⋅⋅

⋅

()()

V

OUT

= 5V
2A AT 12%
DUTY CYCLE

OUT
OUT

IN

IN

0.1µF

47µF

V

IN

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

0.1µF

47µF

V

IN

2.7V TO 6V

R

CHG

40.2k

10µH

2000µF

R2
187k

R1

61.9k

LX1
LX1

LX2
LX2

ON

ON

OFF

REF

CHG

FB

AGND

PGND

PGND

V

OUT =

5V
2A AT 12%
DUTY CYCLE

MAX1688

Figure 7. MAX1688 Typical Application Circuit (GSM Pulsed Load)

V

OUT =

5V

350mA

OUT
OUT

IN

IN

0.1µF

47µF

V

IN

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

87OUT

LX2LX1

LIM [CHG]

IN

SO

TOP VIEW

[ ] ARE FOR MAX1688

3

4

6

5

MAX1687
MAX1688

TSSOP.EPS