SC120 SEMTECH [Semtech Corporation], SC120 Datasheet - Page 13

• Manufacturer Part Number: SC120
• Description: Low Voltage Synchronous Boost Converter
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information
Detailed Description
The SC120 is a synchronous step-up Pulse Width
Modulated (PWM) DC-DC converter utilizing a 1.2MHz
fi xed frequency current mode architecture. It is designed
to provide output voltages in the range 1.8V to 4.0V from
an input voltage as low as 0.7V, with a (output unloaded)
start up input voltage of 0.85V.
The device operates in two modes: PWM and automatic
PSAVE mode. In PWM operation, the devices uses pulse
width modulation control to regulate the output under
moderate to heavy load conditions. It switches to PSAVE
mode when lightly loaded. Quiescent current consump-
tion is as little as 50μA, into the OUT pin, when in PSAVE
mode.
The regulator control circuitry is shown in the Block
Diagram. It is comprised of a programmable feedback
controller, an internal 1.2MHz oscillator, an n-
channel Field Eff ect Transistor (FET) between the LX and
GND pins, and a p-channel FET between the LX and OUT
pins. The current fl owing through both FETs is monitored
and limited as required for startup, PWM operation, and
PSAVE operation. An external inductor must be connected
between the IN pin and the LX pin. When the n-channel
FET is turned on, the LX pin is internally grounded, con-
necting the inductor between IN and GND. This is called
the on-state. During the on-state, inductor current fl ows
to ground and is increasing. When the n-channel FET is
turned off and the p-channel FET is turned on (known as
the off -state), the inductor is then connected between IN
and OUT. The (now decreasing) inductor current fl ows
from the input to the output, boosting the output voltage
above the input voltage.
Output Voltage Selection
The SC120 output voltage can be programmed to an
internally preset value or it can be programmed with
external resistors. The output is internally programmed to
3.3V when the FB pin is connected to GND. Any output
voltage in the range 1.8V to 4.0V can be programmed with
a resistor voltage divider between OUT and the FB pin as
shown in Figure 1.
PRELIMINARY
The values of the resistors in the voltage divider network
are chosen to satisfy the equation:
The value of R
Otherwise, the values of R
desired to achieve low quiescent current.
PWM Operation
The PWM cycle runs at a fi xed frequency (f
with a variable duty cycle (D). PWM operation continu-
ally draws current from the input supply (except for dis-
continuous mode, described below). During the on-state,
of the PWM cycle, the n-channel FET is turned on, ground-
ing 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
fl ow from the input supply through the inductor into the
output capacitor 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 SC120 PWM
controller constrains the value of D such that 0.15 < D < 0.9,
(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
V
OUT
Figure 1 — Output Voltage Feedback Circuit
V
FB
OUT
LX
FB
2
1
should be 590kΩ or larger for stability.
R
R
1
2
IN
/V
1
OUT
and R
R
R
1
), but must be greater in
2
2
can be as large as
C
(optional)
FB
osc
= 1.2MHz),
SC120
13