SC121 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC121' PDF : 23 Pages View PDF
SC121
Applications Information
Detailed Description
The SC121 is a synchronous step-up fixed frequency Pulse
Width Modulated (PWM) DC-DC converter utilizing a
1.2MHz fixed 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. Quiescent
current consumption is typically 3.5mA, entirely into the
OUT pin during boost regulation. (See footnote 1 of the
Electrical Characteristics table.)
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 Effect Transistor (FET) between the LX and
GND pins, and a p-channel FET between the LX and OUT
pins. The current flowing through both FETs is monitored
and limited as required for startup and PWM operation.
An external inductor must be connected between the IN
pin and the LX pin.
Output Voltage Selection
The SC121 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
VOUT
1.191u
¨¨©§1
R1
R2
¸¸¹·
V
A large value of R , ideally 590kΩ or larger, is preferred for
2
stability for V within approximately 400mV of V . For
IN
OUT
lower V , lower resistor values can be used. The values of
IN
R and R can be as large as desired to achieve low quies-
1
2
cent current. C = 22pF is recommended to improve
FB
transient response.
The Enable Pin
The EN pin is a high impedance logical input that can be
used to enable or disable the SC121 under processor
control. V < 0.2V will disable regulation, set the LX pin
EN
in a high-impedance state (turn off both FET switches),
and turn on an active discharge device to discharge the
output capacitor via the OUT pin. Synchronous rectifier
(p-channel FET) bulk switching prevents pass-through
conduction from LX to OUT while disabled. V > 0.85V
EN
will enable the output. The startup sequence from the
EN pin is identical to the startup sequence from the appli-
cation of input power.
L1
IN
LX
EN
OUT
VOUT
R1
CFB
CIN
GND
FB
COUT
SC121
R2
Figure 1 — Output Voltage Feedback Circuit
16
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