SC121ULTRT Semtech, SC121ULTRT Datasheet - Page 16

• 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
Detailed Description
The SC121 is a synchronous step-up fi xed frequency 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. 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 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 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.
C
IN
Figure 1 — Output Voltage Feedback Circuit
IN
EN
GND
SC121
L1
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. C
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
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 rectifi er
(p-channel FET) bulk switching prevents pass-through
conduction from LX to OUT while disabled. V
will enable the output. The startup sequence from the
EN pin is identical to the startup sequence from the appli-
cation of input power.
1
and R
OUT
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
EN
R
R
191
1
2
< 0.2V will disable regulation, set the LX pin
IN
within approximately 400mV of V
FB
2
, ideally 590kΩ or larger, is preferred for
1
= 22pF is recommended to improve
C
R
R
1
2
FB
V
C
OUT
V
OUT
SC121
EN
> 0.85V
OUT
. For
16