ISL97649A

FN7928 Rev 3.00

Page 13 of 22

June 27, 2013

Gate Pulse Modulator Circuit

The gate pulse modulator circuit functions as a three way

multiplexer, switching VGHM between ground, GPM_LO and VGH.

Voltage selection is provided by digital inputs VDPM (enable) and

VFLK (control). High to low delay and slew control is provided by

external components on pins CE and RE, respectively.

When VDPM is LOW, the block is disabled and VGHM is

grounded. When the input voltage exceeds UVLO threshold,

VDPM starts to drive an external capacitor. Once VDPM exceeds

1.215V, the GPM circuit is enabled, and the output VGHM is

determined by VFLK, RESET signal and VGH voltage. If RESET

signal is high and VFLK is high, VGHM is pulled to VGH. When

VFLK goes low, there is a delay controlled by capacitor CE,

following which, VGHM is driven to GPM_LO, with a slew rate

controlled by resistor RE. Note that GPM_LO is used only as a

reference voltage for an amplifier, and thus does not have to

source or sink a significant DC current.

Low to high transition is determined primarily by the switch

resistance and the external capacitive load. High to low transition

is more complex. Take the case where the block is already

enabled (VDPM is H). When VFLK is H, if CE is not externally

pulled above threshold voltage 1, pin CE is pulled low. On the

falling edge of VFLK, a current is passed into pin CE to charge the

external capacitor up to threshold voltage 2, providing a delay

which is adjustable by varying the capacitor on CE. Once this

threshold is reached, the output starts to be pulled down from

VGH to GPM_LO. The maximum slew current is equal to 500/(RE

load capacitance applied to VGHM. The slew rate reduces as

VGHM approaches GPM_LO.

If CE is always pulled up to a voltage above threshold 1, zero

delay mode is selected; thus, there will be no delay from FLK

falling to the point where VGHM starts to fall. Slew down currents

will be identical to the previous case.

At power-down, when VIN falls to UVLO, VGHM will be tied to VGH

until the VGH voltage falls to 3V. Once the VGH voltage falls below

3V, VGHM will not be actively driven until VIN is driven. Figure 14

VGH/VGL Charge Pump

To provide VGH and VGL rails for the application, two external

charge pumps driven by AVDD and the boost switching node can

be used to generate the desired VGH and VGL, as shown in the

“Application Diagram” on page 2.

The number of charge pump stages can be calculated using

Equations 9 and 10.

Where N is the number of charge pump stages and Vd is the

forward voltage drop of one Schottky diode used in the charge

pump. Vd varies with forward current and ambient temperature,

so it should be the maximum value in the diode datasheet

according to max forward current and lowest temperature in the

application condition.

Once the number of the charge pump stages is determined, the

maximum current that the charge pump can deliver can be

calculated using Equations 11 and 12:

Where Freq is the switching frequency of the AVDD boost, C_fly is

the flying capacitance (C8, C10, C11 in the application diagram).

between minimum flying capacitance and VGL and VGH loadings

are shown in Figures 15 and 16. The flying capacitance must be

higher than the minimum value shown in Figures 15 and 16 for a

certain loading on VGL and VGH.

VGH_M is forced to

VGH when RESET

goes to low AND

VGH>3V

Power on delay time

is controlled by

Slope is controlled

by RE

Delay time is controlled

by CE

FIGURE 14. GATE PULSE MODULATOR TIMING DIAGRAM

VIN

UVLO

0

VGH

RESET

VDPM

VFLK

VGHM

THRESHOLD

VGH

GPM_LO

POWER-ON DELAY TIME

SLOPE IS

CONTROLLED BY RE

DELAY TIME IS

CONTROLLED BY CE

VGHM IS FORCED

TO VGH WHEN VIN

1.215V

FALLS TO UVLO AND

VGH>3V

VGL_headroom

NAVDD 2NVd

–

VGL

0



–

=

(EQ. 9)

(EQ. 10)

VGH_headroom

N 1

+

AVDD 2NVd

–

VGH 0



–

=

VGL

N AVDD 2Vd





++

–



=

(EQ. 11)

VGH

AVDD N AVDD 2Vd

–

–

FreqC_fly







+

=

(EQ. 12)