ISL6754

FN6754 Rev 2.00

Page 13 of 20

June 2, 2016

half-cycle. If the average current limit circuitry is not used, a

latching overcurrent shutdown method using external

components is recommended.

The CS to output propagation delay is increased by the Leading

Edge Blanking (LEB) interval. The effective delay is the sum of

the two delays and is 130ns maximum.

Voltage Feed Forward Operation

Voltage feed forward is a technique used to regulate the output

voltage for changes in input voltage without the intervention of

the control loop. Voltage feed forward is implemented in

voltage-mode control loops, but is redundant and unnecessary in

peak current-mode control loops.

Voltage feed forward operates by modulating the sawtooth ramp

in direct proportion to the input voltage. Figure 11 demonstrates

the concept.

Input voltage feed forward may be implemented using the RAMP

input. An RC network connected between the input voltage and

ground, as shown in Figure 12, generates a voltage ramp whose

charging rate varies with the amplitude of the source voltage. At

the termination of the active output pulse, RAMP is discharged to

ground so that a repetitive sawtooth waveform is created. The

RAMP waveform is compared to the VERR voltage to determine

duty cycle. The selection of the RC components depends upon

the desired input voltage operating range and the frequency of

the oscillator. In typical applications, the RC components are

selected so that the ramp amplitude reaches 1.0V at minimum

input voltage within the duration of one half-cycle.

The charging time of the ramp capacitor is:

For optimum performance, the maximum value of the capacitor

should be limited to 10nF. The maximum DC current through the

resistor should be limited to 2mA maximum. For example, if the

oscillator frequency is 400kHz, the minimum input voltage is

300V, and a 4.7nF ramp capacitor is selected, the value of the

resistor can be determined by rearranging Equation 7.

Where t is equal to the oscillator period minus the dead time. If

the dead time is short relative to the oscillator period, it can be

ignored for this calculation.

If feed forward operation is not desired, the RC network may be

resistor divider from CTBUF may be used as the sawtooth signal.

Regardless, a sawtooth waveform must be generated on RAMP

as it is required for proper PWM operation.

Gate Drive

The ISL6754 outputs are capable of sourcing and sinking 10mA

(at rated VOH, VOL) and are intended to be used in conjunction

with integrated FET drivers or discrete bipolar totem pole drivers.

The typical on resistance of the outputs is 50Ω.

FIGURE 11. VOLTAGE FEED FORWARD BEHAVIOR

VIN

ERROR VOLTAGE

RAMP

CT

OUTLL, LR

RAMP

GND

1

2

4

3

5

6

7

8

19

20

11

12

13

14

15

16

ISL6754

9

10

17

18

VIN

FIGURE 12. VOLTAGE FEED FORWARD CONTROL

1





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

–







ln



–

=

S

(EQ. 7)

t

–

1





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

–







ln



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

2.5

–

10

6

–



4.7 10

9

–

1

1

300

----------

–





ln



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

==

159

=

k



(EQ. 8)