SC121ULTRT Semtech, SC121ULTRT Datasheet - Page 17

• Manufacturer Part Number: SC121ULTRT
• Description: IC BOOST SYNC 1.2A MLPD-6
• Manufacturer: Semtech
• Type: Step-Up (Boost)r

Specifications of SC121ULTRT

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 3.3V, 1.8 ~ 5 V
• Current - Output: 1.2A
• Frequency - Switching: 1.2MHz
• Voltage - Input: 0.7 ~ 4.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 6-MLPD-UT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC121ULTRTTR

Applications Information (continued)
PWM Operation
The PWM cycle runs at a fi xed frequency (f
with a variable duty cycle (D). PWM operation continually
draws current from the input supply, except for low
output loads in which current fl ows 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 fl owing
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 rectifi er) 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
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
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 fi nite
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.
ON
= D×T, T = 1/f
OSC
). Therefore, the instantaneous induc-
IN
/V
OUT
), but must be greater in
osc
= 1.2MHz),
The minimum on-time limitation imposes a minimum
boost ratio, so if V
approximately), V
value for a suffi ciently small output load. A higher output
load requires a higher duty cycle to overcome dissipative
losses, such that regulation at programmed V
eventually be restored. But this regulation-restoration
load rises rapidly with V
benefi cially exploited in only rare circumstances. If opera-
tion with high V
using the SC120, a pin compatible dual mode (PWM/
PSAVE) boost converter. The SC120 will support zero load
in PSAVE mode for V
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
approaches V
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, fi xed 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 V
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 V
lation can be maintained for V
the maximum duty cycle and peak current limits. The
OUT
is within 2% of the programmed regulation
IN
IN
(but is still below 1.7V), the n-channel
and low load is required, please consider
IN
OUT
is too close to V
IN
up to V
will rise above the programmed
IN
, so this phenomenon can be
OUT
IN
as low as 0.7V, subject to
+ 150mV.
OUT
(V
IN
IN
and output regu-
> V
OUT
SC121
– 400mV,
OUT
will
OUT
17