SC121 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC121' PDF : 23 Pages View PDF
SC121
Applications Information (continued)
PWM Operation
The PWM cycle runs at a fixed frequency (f = 1.2MHz),
osc
with a variable duty cycle (D). PWM operation continually
draws current from the input supply, except for low
output loads in which current flows periodically from, and
back into, the input. During the on-state of the PWM
cycle, the n-channel FET is turned on, grounding the
inductor at the LX pin. This causes the current flowing
from the input supply through the inductor to ground to
ramp up. During the off-state, the n-channel FET is turned
off and the p-channel FET (synchronous rectifier) is turned
on. This causes the inductor current to flow from the
input supply through the inductor into the output capaci-
tor and load, boosting the output voltage above the input
voltage. The cycle then repeats to re-energize the
inductor.
Ideally, the steady state (constant load) duty cycle is
determined by D = 1 – (V /V ), but must be greater in
IN OUT
practice to overcome dissipative losses. The SC121 PWM
controller constrains the value of D such that 0.20 < D < 0.90
(approximately).
The average inductor current during the off-state multi-
plied by (1-D) is equal to the average load current. The
inductor current is alternately ramping up (on-state) and
down (off-state) at a rate and amplitude determined by
the inductance value, the input voltage, and the on-time
(T = D×T, T = 1/f ). Therefore, the instantaneous induc-
ON
OSC
tor current will be alternately larger and smaller than the
average.
If the average output current is sufficiently small, the
minimum inductor current can ramp down to zero during
the off-state. Discontinuous mode operation (where both
FETs turn off as the inductor current reaches zero) is not
supported in the SC121, since this would result in a finite
positive minimum current from input to output, which
would cause an uncontrolled rise in output voltage in this
case. Instead, the inductor current will reverse for the
remainder of the off-state, flowing from the output
capacitor into the OUT pin, through the p-channel FET to
the LX pin, and through the inductor to the input capaci-
tor. Negative inductor current ripple allows regulation
even with zero output load. The energy returned to the
input capacitor is not wasted, but dissipative conduction
losses will inevitably occur.
The minimum on-time limitation imposes a minimum
boost ratio, so if V is too close to V (V > V – 400mV,
IN
OUT IN OUT
approximately), V will rise above the programmed
OUT
value for a sufficiently small output load. A higher output
load requires a higher duty cycle to overcome dissipative
losses, such that regulation at programmed V will
OUT
eventually be restored. But this regulation-restoration
load rises rapidly with V , so this phenomenon can be
IN
beneficially exploited in only rare circumstances. If opera-
tion with high V and low load is required, please consider
IN
using the SC120, a pin compatible dual mode (PWM/
PSAVE) boost converter. The SC120 will support zero load
in PSAVE mode for V up to V + 150mV.
IN
OUT
Regulator Startup, Short Circuit Protection,
and Current Limits
The SC121 permits power up at input voltages from 0.85V
to 4.5V. Soft-start startup current limiting of the internal
switching n-channel and p-channel FET power devices
protects them from damage in the event of a short
between OUT and GND. As the output voltage rises, pro-
gressively less-restrictive current limits are applied. This
protection unavoidably prevents startup into an exces-
sive load.
Upon enable, the p-channel FET between the LX and OUT
pins turns on with its current limited to approximately
150mA, the short-circuit output current. When V
OUT
approaches V (but is still below 1.7V), the n-channel
IN
current limit is set to 350mA (the p-channel limit is dis-
abled), the internal oscillator turns on (approximately
200kHz), and a fixed 75% duty cycle PWM operation
begins. (See the section PWM Operation.) When the
output voltage exceeds 1.7V, fixed frequency PWM opera-
tion begins, with the duty cycle determined by an n-
channel FET peak current limit of 350mA. When this
n-channel FET startup current limit is exceeded, the on-
state ends immediately and the off-state begins. This
determines the duty cycle on a cycle-by-cycle basis.
When VOUT is within 2% of the programmed regulation
voltage, the n-channel FET current limit is raised to 1.2A,
and normal voltage regulation PWM control begins.
Once normal voltage regulation PWM control is initiated,
the output becomes independent of VIN and output regu-
lation can be maintained for V as low as 0.7V, subject to
IN
the maximum duty cycle and peak current limits. The
17
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