SC21150 Datasheet, PDF (15/20 Pages) Semtech Corporation – Tiny 1.2A Synchronous Buck Regulator

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 16 switch-
ing cycles before the controller will return to PFM
operation.
Soft start
The SC21150 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 100usec. 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 SC21150 includes an under-voltage lockout (UVLO)
to inhibit switching until the input voltage exceeds 2.16V.
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 SC21150 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 SC21150 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.
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