SC3203SETRC datasheet(16/20 Pages) SEMTECH

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SC3203

Applications Information (continued)

has passed a new soft start cycle is attempted. After soft

start, if the output is still in a fault condition the switcher

will again shut down and wait another 32msec before

attempting the next soft-start. The 32msec delay between

soft start cycles reduces power loss and heating in the

power components.

Note that an external VIN UVLO event is treated internally

as a fault condition and triggers the Hiccup Mode feature.

This will lead to a delay on restart when VIN has recovered

to a normal level. The EN pin can rise above the VIN UVLO

threshold but the controller must complete a 32msec

time-out before the switcher will start up. The same delay

also occurs for OVP and Over-Temperature shut down.

BYP Regulator

The SC3203 has an internal regulator that supplies the

bias voltage for the PWM controller. Although this voltage

is available externally, the BYP regulator is designed for

internal use only. The BYP pin requires a 1μF bypass

capacitor. When the EN pin exceeds typically 1V, the BYP

regulator is enabled and goes through a start-up

sequence.

During start-up while the BYP output voltage remains

below 4V, the BYP short-circuit protection is active and

limits the current to typically 35mA. After BYP exceeds

4.0V the LDO operates in voltage regulation mode with

output current limited to typically 100mA (see Figure 9).

Short-circuit Protection 35m A

Voltage regulating with

100m A current lim it

BYP Voltage (V)

Figure 9 — LDO Start-Up

BYP UVLO and POR

The BYP UVLO (Under-Voltage Lock-Out) circuitry inhibits

switching and tri-states the power mosfets until the BYP

voltage rises above 4.0V. An internal POR (Power-On Re-

set) occurs when BYP exceeds 4.0V, which resets the fault

latch and enables the soft start ramp. The SC3203 then

begins a soft start cycle. The PWM will shut off if BYP falls

below 3.7V.

Boost Supply

The Boost supply provides bias for the high-side mosfet

driver. Connect a 10nF between Boost and LX. Larger

values of Boost capacitance should not be used, the

Boost driver circuit is designed for 10nF.

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