MAX1763(2001) Ver la hoja de datos (PDF) - Maxim Integrated

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MAX1763
(Rev.:2001)

1.5A, Low-Noise, 1MHz, Step-Up DC-DC Converter

Maxim Integrated 

1.5A, Low-Noise, 1MHz, Step-Up

DC-DC Converter

in dropout, the MOSFET on-resistance will determine

the dropout voltage (VDROPOUT = IOUT ✕ RDS(ON)). If a

lower RDS(ON) PFET is used, increase the linear-regula-

tor output filter capacitance to maintain stability.

The output capacitance can be determined by the

function:

COUT ≥ [ (VREF / VOUT) ✕ GM ✕ GFS ✕ CG ✕ (RG ✕ 2) ]

and

COUT ≥ 10 ✕ [ (VREF / [VOUT ✕ GBP]) ✕ GM ✕ GFS ✕ RG ]

where VREF is the 0.983V reference voltage, GM is the

10mS internal amplifier transconductance, GFS is the

external MOSFET transconductance, RG is the gate-

source resistor, and GBP is the gain-bandwidth prod-

uct of the internal gain block, 63Mrad/s.

___________________Design Procedure

Setting the Output Voltage

For a fixed 3.3V output, connect FB to GND. To set the

output voltage between 2.5V and 5.5V, connect a resis-

tor voltage-divider to FB from OUT to GND (Figure 6).

The input bias current into FB is less than 100nA, allow-

ing large-value divider resistors without sacrificing

accuracy. Connect the resistor voltage-divider as close

to the IC as possible, within 0.2in (5mm) of FB. Choose

R2 of 30kΩ or less, then calculate R1 using:

R1 = R2((VOUT / VFB ) - 1)

where VFB, the boost-regulator feedback set point, is

1.245V.

Setting the Switch Current Limit

and Soft-Start

The ISET pin adjusts the inductor peak current and can

also be used to implement soft-start. With ISET con-

nected to REF, the inductor current limits at 2.5A. With

ISET connected to a resistive divider set from REF to

GND, the current limit is reduced according to:

ILIM = 2.5(VISET / 1.25) [A]

Implement soft-start by placing a resistor from ISET to

REF (>300kΩ) and a capacitor from ISET to GND. In

shutdown, ISET is discharged to GND through an inter-

nal 100kΩ resistor. As the capacitor voltage rises, the

output current is allowed to increase, and the output

voltage rises. The speed at which the output rises is

determined by the soft-start time constant:

tSS = RSS CSS

where RSS ≥ 300k.

Both features may be implemented simultaneously by

placing a capacitor across the lower resistor of the cur-

rent-limiting resistive divider (Figures 7 and 8).

OUT

R1

MAX1763

FB

R2

( ) R1 = R2

VOUT

VFB

-

1

, VFB = 1.245V, R2 ≤ 30k

Figure 6. Connecting Resistors for External Feedback

0.22µF

RSS

REF

MAX1763

ISET

CSS

ILIM = 2.5A

tSS = RSS CSS

Figure 7. Soft-Start with Maximum Switch Limit Current

0.22µF

RSS1

REF

MAX1763

ISET

RSS2 CSS

( ) ILIM = 2.5A

RSS2

RSS1 + RSS2

tSS = (RSS1 RSS2) CSS

Figure 8. Soft-Start with Reduced Switch Limit Current

Package Selection

The MAX1763 is available in two packages, a 16-pin

QSOP and a 16-pin TSSOP-EP. Since the MAX1763

has excellent efficiency, most applications are well

served by the QSOP package. If the application

requires high power dissipation, or operation in a high

ambient temperature, choose the TSSOP-EP package.

The TSSOP-EP is equipped with an exposed metal pad

on its underside for soldering to grounded circuit board

copper. This reduces the junction-to-case thermal

12 ______________________________________________________________________________________

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