SC196A Datasheet, PDF (7/17 Pages) Semtech Corporation – 1.5A Synchronous Buck Converter with Integrated Power Devices

SC196A
POWER MANAGEMENT
Applications Information
SC196A Detailed Description
The SC196A is a synchronous step-down, Pulse Width
Modulated (PWM), DC-DC converter utilizing a 1MHz
fixed-frequency current mode architecture. The device
is designed to operate in a fixed-frequency PWM mode
across the full load range and can enter Power Save Mode
(PSAVE), utilizing Pulse Frequency Modulation (PFM) at
light loads to maximize efficiency.
Operation
During normal operation, the PMOS is activated on each
rising edge of the internal oscillator. Current feedback
for the switching regulator uses the PMOS current path,
and it is amplified and summed with the internal slope
compensation network. The voltage feedback loop uses
an internal feedback divider. The on-time is determined by
comparing the summed current feedback and the output
of the error amplifier. The period is set by the onboard
oscillator or by an external clock attached to the SYNC/
PWM pin.
The SC196A has an internal synchronous NMOS rectifier
and does not require a Schottky diode on the LX pin.
Programmable Output Voltage
The SC196A has four pre-determined output voltage
values which can be individually selected by the correct
programming oftheVID0andVID1pins (see Programmable
Output Voltage table on Page 4). This eliminates the need
for external programming resistors, saving PCB area and
inventory. The VID pins can be statically tied to GND or
VIN for fixed output configurations, or they may be driven
by a microprocessor, enabling the possibility of dynamic
voltage adjustment for host equipment "sleep" states.
Continuous Conduction & Oscillator Synchronization
The SC196A is designed to operate in continuous
conduction, fixed-frequency mode. When the SYNC/PWM
pin is tied high, the part runs in PWM mode using the
internal oscillator. The part can be synchronized to an
external clock by driving a clock signal into the SYNC/
PWM pin. The part synchronizes to the rising edge of the
clock.
Protection Features
The SC196A provides the following protection features:
• Thermal shutdown
• Current limit
PRELIMINARY
• Over-voltage protection
• Soft-start
Thermal Shutdown
The device has a thermal shutdown feature to protect
the SC196A if the junction temperature exceeds 145°C.
In thermal shutdown, the on-chip power devices are
disabled, tri-stating the LX output. Switching will resume
when the temperature drops by 10°C. During this time,
if the output voltage decreases by more than 60% of its
programmed value, a soft-start will be invoked.
Current Limit
The PMOS and NMOS power devices of the buck switcher
are protected by current limit functions. In the case of a
short to ground on the output, the part enters frequency
foldback mode, which causes the switching frequency to
divide by a factor determined by the output voltage. This
prevents the inductor current from "stair-casing".
Over-Voltage Protection
Over-voltage protection is provided on the SC196A. In
the event of an over-voltage on the output, the PWM drive
is disabled, tri-stating the LX output. The part will not
resume switching until the output voltage has fallen 2%
below the regulation voltage.
Soft-Start
The soft-start mode is enabled after every shutdown cycle
to limit in-rush current. In conjunction with the frequency
foldback, this controls the maximum current during start-
up. The PMOS current limit is stepped up through seven
soft-start levels to the full value by a timer driven from
the internal oscillator. During soft-start, the switching
frequency is stepped by 1/8, 1/4, and 1/2 of the internal
oscillator frequency up to the full value, under control of
three output voltage thresholds. As soon as the output
voltage is within 2% of the regulation voltage, soft-start
mode is disabled.
Power Save Mode Operation
The PSAVE mode may be selected by tying the SYNC/
PWM pin to GND. Selecting PSAVE mode will enable the
SC196A to automatically activate/deactivate operation
at light loads, maximizing efficiency across the full load
range. The SC196A automatically detects the load current
at which it should enter PSAVE mode. The SC196A is
optimized to track maximum efficiency with respect to
V_IN.
 2006 Semtech Corp.

www.semtech.com