SC3203 datasheet(15/20 Pages) SEMTECH

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SC3203

Applications Information (continued)

When the EN input is below 1.5V, the switcher is off.

mosfet Q1 is on and resistor REN is connected to the EN

pin. When the EN pin reaches 1.5V, the comparator out-

put drives high and the switcher is enabled. mosfet Q1

is then switched off, removing R

EN from the circuit which

immediately raises the voltage at the EN pin and pro-

vides VIN hysteresis. An additional hysteresis of 100mV is

provided internally at the comparator input.

The equations for selecting the VIN_ON and VIN_OFF

thresholds are shown below. Note that R

EN has a typical

value of 810kΩ and tolerance of +/- 20%.

Soft Start of PWM Regulator

Soft start is achieved in the PWM regulator by ramping

the internal FB Comparator reference from zero to 750mV.

When the ramp voltage reaches 750mV, the ramp is

ignored and the FB comparator switches over to a fixed

750mV threshold. During soft start the FB pin follows the

internal ramp, which limits the start-up inrush current

and provides a controlled soft start profile for a wide

range of applications. Typical soft start ramp time is

1.8msec.

During soft start the regulator turns off the low-side

mosfet during any cycle if the inductor current falls to

zero. This prevents negative inductor current, allowing the

device to start into a pre-biased output with negligible

drooping on the output.

Power Good Output

The Power Good (PGood) output is an open-drain indica-

tor. The output is open (high impedance) when the

output is within normal regulation. During startup the

PGood output is held low at AGND while the output

ramps up. If the output reaches normal levels, the PGood

output will switch to high (open circuit) typically 3.3msec

after the EN pin drives high to begin a soft start cycle.

On startup, the FB rising edge threshold for PGood is

750mV -10% (675mV). Once PGood drives high, the FB

pin must fall to 750mV -15% (638mV) before the PGood

output will drive low.

Output Over-Voltage Protection

OVP (Over-Voltage Protection) becomes active as soon as

the switcher is enabled. The OVP threshold is set at 750mV

+ 20% (900mV). When the FB pin exceeds the 900mV the

low-side mosfet turns on. It will remain on while the low-

side current is negative (remain on while current flows out

of the LX pin). When the low-side current reaches zero or

becomes positive (current into the LX pin), the low-side

mosfet turns off and the LX pin is tri-stated. LX remains

tri-stated until the FB pin falls below the 750mV +15%

(863mV), which starts the Hiccup Mode timer and forces a

32msec delay before a new soft start cycle begins.

The PGOOD output also drives low when the FB pin

exceeds the OVP threshold.

Output Under-Voltage Protection

When the FB pin falls to 75% of its nominal voltage (falls

to 563mV) for eight consecutive clock cycles, the mosfets

are turned off and the controller enters Hiccup Mode

operation. Under-Voltage faults are normally caused by

an output overload; the controller will automatically

recover on the next soft start cycle after the overload is

removed.

Over-Temperature Protection

If the internal temperature rises to 160°C the device will

shut down. The device remains off until the temperature

drops to 150°C, which starts the Hiccup Mode timer and

forces a 32msec delay before a new soft start cycle

begins.

Hiccup Mode (Automatic Fault Recovery)

The SC3203 includes Hiccup Mode fault protection. If the

switcher output shuts down due to a fault condition, the

device remains off until the fault condition is removed,

which begins the 32ms Hiccup Mode timer. After 32msec

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