L6986H datasheet(41/68 Pages) STMICROELECTRONICS

41 / 68 page

Application notes

6.1

Output voltage adjustment

The error amplifier reference voltage is 0.85 V typical.

The output voltage is adjusted accordingly as per equation below: (see Figure 42. L6986H application circuit).

VOUT=0.85⋅(1+R1R2)

(40)

Cr1 capacitor is sometimes useful to increase the small signal phase margin (please refer to Compensation

network design).

Figure 42. L6986H application circuit

L6986H

Power GND

signal GND

Cr1

uC RST

RST

VCC

SS/INH

SYNCH

FSW

MLF

COMP 7

DELAY

FB 9

SGND

PGND PGND

LX 13

VIN

VBIAS

GND

VIN

VOUT

6.2

Switching frequency

A resistor connected to the FSW pin features the selection of the switching frequency. The pinstrapping is

performed at power-up, before the soft-start takes place. The FSW pin is pinstrapped and then driven floating in

order to minimize the quiescent current from VIN. Please refer to Table 2 to identify the pull-up / pull-down resistor

value. fSW = 250 kHz / fSW = 500 kHz preferred codifications do not require any external resistor.

6.3

MLF pin

A resistor connected to the MLF pin features the selection of the between low noise mode / low consumption

mode and the different RST thresholds. The pinstrapping is performed at power-up, before the soft-start takes

place. The FSW pin is pinstrapped and then driven floating in order to minimize the quiescent current from VIN.

Please refer to Table 7. LNM/ LCM selection (L6986H3V3), Table 8. LNM/ LCM selection (L6986H5V), and

Table 9. LNM/ LCM selection (L6986H) to identify the pull-up / pull-down resistor value. (LNM, RST threshold

93%) / (LCM, RST threshold 93%) preferred codifications don't require any external resistor.

6.4

Voltage supervisor

The embedded voltage supervisor (composed of the RST and the DELAY pins) monitors the regulated output

voltage and keeps the RST open collector output in low impedance as long as the VOUT is out of regulation. In

order to ensure a proper reset of digital devices with a valid power supply, the device can delay the RST assertion

with a programmable time.

L6986H

Application notes

DS12905 - Rev 2

page 41/68

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