ISL6740A

11

FN9195.3

February 9, 2012

The hysteresis voltage,

ΔV, is:

Setting R3 equal to zero results in the minimum hysteresis,

and yields:

Output voltage variation caused by changes in the supply

voltage may be virtually removed through a technique known

as feed forward compensation. Using feed forward, the duty

cycle is directly modulated based on changes in the input

voltage only. No closed loop feedback system is required. The

feed forward circuit uses the voltage applied to the UV/FF pin

to modulate the oscillator ramp amplitude with minimal effect

on the switching frequency and deadtime of the oscillator. The

voltage feed forward operates over a 3:1 input voltage range.

The voltage applied to the UV/FF pin is multiplied by 0.8 and

peak threshold voltage. As the voltage applied to UV/FF varies,

the CT peak voltage and the CT charge and discharge currents

vary, all in direct proportion to each other. The result is an

frequency invariant.

voltage on UV increases. The UV threshold defines the

cycle operation.

For unregulated bus converters and DC transformers, feed

forward can compensate for input voltage variations without a

closed loop feedback network. A resistive voltage divider from

example, if the desired duty cycle at the minimum operating

voltage is 90%, then:

Overcurrent Protection

There are two overcurrent protection mechanisms in the

ISL6740A, one for light overcurrent and one for heavy over

load. They are referred to, respectively, as overcurrent

protection and short circuit protection.

OVERCURRENT OPERATION

Overcurrent delayed shutdown is enabled once the soft-start

cycle is complete. If an overcurrent condition is detected, the

soft-start charging current source is disabled and the soft-start

capacitor is allowed to discharge through a 15µA source. At

the same time a 50µs re-triggerable one-shot timer is

activated. It remains active for 50µs after the overcurrent

condition ceases. If the soft-start capacitor discharges by more

then 0.25V to 4.25V, the output is disabled and the Fault signal

asserted. This state continues until the soft-start voltage

reaches 270mV, at which time a new soft-start cycle is

initiated. If the overcurrent condition stops at least 50µs prior

to the soft-start voltage decreasing to 4.25V, the soft-start

charging currents revert to normal operation and the soft-start

voltage is allowed to recover.

Figure 7 shows the overcurrent behavior during SS. Although

an overcurrent condition exists, a shutdown is not allowed prior

to completion of the SS cycle. Only peak current limit operates

during the soft-start cycle. If the overcurrent condition were to

continue beyond the soft-start cycle, a delayed overcurrent

shutdown would occur as shown in Figure 8.

FIGURE 5. UV HYSTERESIS

R1

R2

R3

1.00V

10

μA

ON

+

-

()

R1 R2

+

R2

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

=

V

(EQ. 7)

ΔV10

5

–

R1 R3

R1 R2

+

R2

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

⎝⎠

⎛⎞

•

+

〈〉

•

=

V

(EQ. 8)

ΔV10

5

–

R1

•

=

V

(EQ. 9)

()

+

=

V

(EQ. 10)

FIGURE 6. FEED FORWARD BEHAVIOR

V

UV/FF

V

ERROR

CT

OUTA

OUTB

⁄

0.8

•

() 0.8

+

=

V

0.9 1.0 0.8

•

() 0.8

+

1.52

=

=

V

(EQ. 11)

SS

0.6 V OC

CS

OUTA

OUTB

4.5 V

FIGURE 7. PULSE-BY-PULSE OC BEHAVIOR DURING SS