11

FN9195.0

February 7, 2005

UV/Inhibit fault; otherwise, it is inactive and does not affect

the node voltage. The magnitude of the hysteresis is a

function of the external resistor divider impedance. If the

resistor divider impedance results in too little hysteresis, a

series resistor between the UV pin and the divider may be

used to increase the hysteresis. A soft-start cycle begins

when the UV/Inhibit fault clears.

The voltage hysteresis created by the switched current

source and the external impedance is generally small due to

the large resistor divider ratio required to scale the input

voltage down to the UV threshold level. A small capacitor

placed between the UV input and ground may be required to

filter noise out.

The hysteresis voltage,

∆V, 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

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 amplitude modulated sawtooth waveform on

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

For 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 15

µA

source. At the same time a 50

µs re-triggerable one-shot

timer is activated. It remains active for 50

µs 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

FIGURE 5. UV HYSTERESIS

R1

R2

R3

1.00V

10

µA

ON

+

-

()

R1 R2

+

R2

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

=

V

(EQ. 7)

∆V10

5

–

R1 R3

R1 R2

+

R2

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





•

+

〈〉

•

=

V

(EQ. 8)

∆V10

5

–

R1

•

=

V

(EQ. 9)

()

()

∆V

+

=

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)

ISL6740A