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

IPEAK

SET BY

VLIM (VCHG)

IPEAK - 200mA

HYSTERESIS

BAND

ILOAD

VOUT

“ON”

CONTROL INPUT

TIME

( ) ARE FOR MAX1688

Figure 2. Hysteretic Inductor Current

VOUT

OUT

MAX1687

MAX1688

R1

FB

R2

( ) R1 = R2

VOUT - VFB

VFB

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).

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).

TIME

Figure 4. Timing Diagram of “ON”

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 · (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.

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