SC21150ACSTRT datasheet(15/20 Pages) SEMTECH

15 / 20 page

SC21150

Applications Information (continued)

Power Save (PFM) Operation

PFM operation is enabled when the FPWM input is a logic

low. Under this condition, the IC automatically reduces

switching frequency under light load conditions to con-

serve power. In PFM operation, the FB ripple signal is

routed to a gain stage and compared to an internal refer-

ence. A switching cycle is as follows.

On-time: The high-side mosfet turns on, as trig-

gered by the FB signal falling to the internal ref-

erence. The high-side stays on for a

predetermined time based on VIN and VOUT,

then turns off.

Off-time: the low-side mosfet turns on. Inductor

current is monitored via the low-side switch.

When the current falls to zero, the low-side

mosfet turns off.

Dead-time: both mosfets are off. If the FB signal

is higher than the internal reference, both

mosfets remain off. During the dead-time the

load current is supplied by the output capacitor,

causing the output and the FB signal to fall.

When the FB signal falls to the 800mV threshold,

the next high-side pulse begins.

As load current increases, the dead-time between pulses

reduces. On any cycle, if the minimum off-time (60nsec)

has elapsed and the FB signal is calling for another high-

side on-time, the controller exits PFM and enters PWM

operation.

Once the controller is in PWM operation, the inductor

current must go below zero (go negative) for 6 switch-

ing cycles before the controller will return to PFM

operation.

Soft start

The SC250 contains an internal soft start to reduce

inrush current during start-up. During start-up, the FB

signal is compared to an internal ramp, providing a

gradual ramp on VOUT. The timing to move VOUT from

0V to 95% of final value is typically 00usec. Optional soft

start times of 200usec, 400usec, and 800usec can be

provided.

•

Pre-bias Start-up

During start-up, the low-side mosfet turns off if the induc-

tor current falls negative (into the LX pin) on any cycle.

This prevents discharge of the output capacitor during

the soft-start cycle. Note that if the output is loaded

during a Pre-Bias start-up, the output voltage may fall

until the internal Soft-start ramp rises above the voltage

seen at the FB pin.

VIN Under-Voltage Lockout

The SC250 includes an under-voltage lockout (UVLO)

to inhibit switching until the input voltage exceeds 2.6V.

The VIN UVLO threshold has 85mV hysteresis to prevent

chatter during start-up.

EN Input

The EN input is a logic level input. When EN is set to logic

high, the SC250 goes through an internal reset to

prepare for operation. The soft-start ramp and mosfet

switching begin typically 50μsec after EN is driven high.

When EN is low (grounded), the power mosfets are off

and the SC250 is its lowest power state. The automatic

discharge mosfet connected to LX is on to discharge the

output before the next start-up cycle.

FPWM Input

The FPWM input is a logic level input that enables or dis-

ables the Power Save feature. Connect the FPWM input

to a logic low or GND to enable Power Save operation. To

disable Power Save operation and provide continuous

switching at all loads, connect the FPWM input to a logic

high or VIN.

Automatic Discharge

When the EN pin is pulled low (grounded), the power

mosfets are tri-stated and the LX pin is connected to GND

through an internal mosfet to discharge the output

capacitors. Typical mosfet RDS

ON is 40 ohms at 5V and

25°C. Note that RDS

ON is a function of both VIN and

temperature.

The automatic discharge feature can optionally be dis-

abled from the device.

SC21150ACSTRT датащи(PDF) 15 Page - Semtech Corporation

SC21150ACSTRT датащи(HTML) 15 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

датащи скачать

ссылки URL