ISL6521 datasheet(7/14 Pages) RENESAS

7 / 14 page

ISL6521

FN9148 Rev 2.00

Page 7 of 14

Feb 8, 2005

1. The maximum rDS(ON) at the highest junction temperature.

2. The minimum IOCSET from the specification table.

3. Determine IPEAK for IPEAK > IOUT(MAX) + (I)/2, where I

is the output inductor ripple current.

For an equation for the ripple current see the section under

component guidelines titled ‘Output Inductor Selection’.

Output Voltage Selection

The output voltage of the PWM converter can be resistor-

programmed to any level between VIN and 0.8V. However,

since the value of RS1 is affecting the values of the rest of the

compensation components, it is advisable its value is kept

between 2k

 and 5k.

Output voltage selection on the linear regulators is set by

means of external resistor dividers as shown in Figure 4. The

two resistors used to set the voltage on each of the three linear

regulators have to meet the following criteria: their value while

in a parallel connection has to be less than 5k

, or otherwise

said, the following relationship has to be met:

To ensure the parallel combination of the feedback resistors

equals a certain chosen value, RFB, use the following

equations:

, where

VOUT - the desired output voltage,

VFB - feedback (reference) voltage, 0.8V.

Application Guidelines

Soft-Start Interval

The soft-start function controls the output voltages rate of rise to

limit the current surge at start-up. The soft-start function is

integrated on the chip and the soft-start interval is fixed.

PWM Controller Feedback Compensation

The PWM controller uses voltage-mode control for output

regulation. This section highlights the design consideration for

a PWM voltage-mode controller. Apply the methods and

considerations only to the PWM controller.

Figure 5 highlights the voltage-mode control loop for a

synchronous-rectified buck converter. The output voltage

(VOUT) is regulated to the reference voltage level, 0.8V. The

error amplifier (Error Amp) output (VE/A) is compared with the

oscillator (OSC) triangular wave to provide a pulse-width

modulated (PWM) wave with an amplitude of VIN at the

PHASE node. The PWM wave is smoothed by the output filter

(LO and CO).

The modulator transfer function is the small-signal transfer

function of VOUT/VE/A. This function is dominated by a DC

Gain, given by VIN/VOSC, and shaped by the output filter, with

a double pole break frequency at FLC and a zero at FESR.

UGATE

OCSET

PHASE

OCC

GATE

CONTROL

VCC

A

VDS(ON)

VSET

ROCSET

VIN = +5V

OVERCURRENT TRIP:

IOCSET

FIGURE 3. OVERCURRENT DETECTION

PWM

DRIVE

iD rDS ON



¥IOCSET ROCSET

VDS VSET

VPHASE VIN VDS

–

VOCSET VIN VSET

–

DRIVE3

FB3

FB4

COUT4

COUT3

ISL6521

VOUT3

VOUT4

+5VIN

DRIVE4

RS3

RP3

RS4

RP4

VOUT 0.8

--------









FIGURE 4. ADJUSTING THE OUTPUT VOLTAGE OF ANY OF

THE FOUR REGULATORS (OUTPUTS 3 AND 4

PICTURED)

RS RP



RS RP

---------------------- 5k





VOUT

VFB

---------------- RFB



RS VFB



VOUT VFB

–

---------------------------------

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