sc120skevb Semtech Corporation, sc120skevb Datasheet - Page 20

• Manufacturer Part Number: sc120skevb
• Description: Low Voltage Synchronous Boost Regulator
• Manufacturer: 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 5.0V 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
C
IN
Figure 1 — Output Voltage Feedback Circuit
IN
EN
GND
SC120
L1
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 5.0V can be programmed
with a resistor voltage divider between OUT and the FB
pin as shown in Figure 1.
The values of the resistors in the voltage divider network
are chosen to satisfy the equation
A large value of R
stability for V
lower V
R
cent current.
1
OUT
and R
V
FB
LX
OUT
IN
2
, lower resistor values can be used. The values of
can be as large as desired to achieve low quies-
1
R
R
2 .
1
2
IN
within approximately 400mV of V
1
2
, ideally 590kΩ or larger, is preferred for
R
R
C
2
1
FB
V
C
.
OUT
V
OUT
SC120
OUT
. For
20