ML4828CS datasheet(8/11 Pages) MICRO-LINEAR

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ML4828

2.5V

V(PIN 20)

V(PIN 12)

If the current limit condition persists, then IRST will be

reactivated, thus charging CRST to a higher level as shown

in Figure 7. Eventually, the voltage at RST will exceed

2.5V, and the soft start comparator will trip, shutting down

all power drivers and inhibiting any further delivery of

power. At the same time, the soft start capacitor CSS is

discharged to prepare for the next start up cycle.

During the ILIM shutdown, IRST is turned off, and CRST is

discharged through RRST with a time constant of:

RST

=×

(12)

When the condition causing the current limit is removed,

RRST will discharge CRST with a time constant of tRST.

When the voltage at RST (pin 12) drops to 1.25V, the soft

start comparator and the converter will undergo a start up

cycle. The restart delay (tD(RST)) is given by:

D RST

RST

()

=× 139

(13)

For example, with CRST = 25µF and RRST = 240kΩ:

500

(14)

and

D RST

()

=Ω × µ ×

240

1 39

8 3

(15)

Since the threshold for shutdown is 2.5V, the controller

will shut down after approximately 125ms. After the

converter recovers form the current limit condition, the

controller will reactivate after 8.3s.

UNDERVOLTAGE LOCKOUT

During start-up, the ML4828 draws very little current

(typically 150

µA) and VREF is disabled. When VCC rises

above 6.0V, the internal circuitry and VREF are enabled,

and will stay enabled until VCC falls below the 4.5V UV

lockout threshold.

SHUTDOWN FUNCTION

The ML4828 can be externally shut down by bringing

SDN (pin 19) low. The shutdown threshold (VSD) is given

=×

033

(16)

For example, if VCC= 5V, then VSD = 1.67V. As long as

2.4V < VCC < 6.0V, the SDN pin will be TTL compatible.

Figure 7. ILIM and Resulting RCRST

Waveforms During Short Circuit.

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