MAX1687ESA 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1687ESA

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

Maxim Integrated 

Step-Up DC-DC Converters with

Precise, Adaptive Current Limit for GSM

Pin Description

PIN

MAX1687

MAX1688

SO

TSSOP

SO

TSSOP

1

1, 2

1

1, 2

2

3, 4

2

3, 4

3

5

—

—

—

—

3

5

4

6

4

6

—

7

—

7

—

8

—

8

5

9

5

9

6

—

6

—

—

10

—

10

—

11, 12

—

11, 12

7

13, 14

7

13, 14

8

15, 16

8

15, 16

NAME

IN

LX1

LIM

CHG

FB

REF

N.C.

ON

GND

AGND

PGND

LX2

OUT

FUNCTION

Supply Voltage Input. Connect Battery to IN. Bypass to GND with

a 47µF 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 out-

put 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

3µA in shutdown mode.

Ground

Analog Ground

Power Ground

N-Channel and P-Channel MOSFET Drain

Output

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.

6 _______________________________________________________________________________________

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