ML4810, ML4811

5

SWITCH CURRENT

DIODE CURRENT

SWITCH CURRENT

DIODE CURRENT

+

–

+

–

–

+

–

+

–

+

–

12

4

9

FROM OSC.

ERROR

AMP

QI

Q2

F2

F3

F1

QI

G1

G2

I(2)

C1

I(1)

I(1)

R

R

Q

A5

A2

A4

A3

A1

4UL0

S

Q

S

R

1.5V

1.1V

S

Q

1.1V

2.45V

20

RESET DELAY

FROM OSC.

FROM PWM

COMPARATOR

SOFT START

V+

C2

R1

FUNCTIONAL DESCRIPTION

SOFT START AND CURRENT LIMIT

The ML4810/11 offers a unique system of fault detection

and reset. Most PWM controllers use a two threshold

method which relies on the buildup of current in the

output inductor during a fault. This buildup occurs

because:

1. Inductor di/dt is a small number when the switch is off

under load fault (short circuit) conditions, since VL is

small.

2. Some energy is delivered to the inductor since the IC

must first detect the over-current because there is a

finite delay before the output switch can turn off.

A method of circumventing this problem involves

“counting” the number of times the controller terminates

the PWM cycle due to the cycle by cycle current limit.

When the switch current crosses the 1.1V threshold A1

signals the F1 to terminate the cycle and sets F3, which is

reset at the beginning of the PWM cycle. The output of F3

turns on a current source to charge C2. When, after

several cycles, C2 has charged to 2.45V, A5 turns on F2 to

discharge soft start capacitor C1. Charge is short lived (for

instance a disk drive start-up or a board being plugged

into a live rack) the control can “ride out” the surge with

the switch protected by the cycle by cycle limit. R1 and

C1 can be selected to track diode heating, or to ride out

various system surge requirements as required.

If the high current demand is caused by a short circuit, the

duty cycle will be short and the output diodes will carry

the current for the majority of PWM cycle. C2 charges

fastest for low duty cycles (since F3 will be on for a longer

time) providing for quicker shutdown during short-circuit

when the output diodes are being maximally stressed.

Figure 1. Current Waveforms for Slow Turn-Off System

with Load Fault

This scheme was adequate for controllers with longer

comparator propagation delays and turn-off delays than is

desirable in a high frequency system. For systems with

low propagation delays, very little energy will be

delivered to the inductor and the current “ratcheting”

described above will not occur. This results in the

controller never detecting the load fault and continuing to

pump full current to the load indefinitely, causing heating

in the output rectifiers and inductor.

Figure 2. Current Waveforms for High Speed System

with Load Fault

Figure 3. Integrating Soft Start Reset