sc202amltrt Semtech Corporation, sc202amltrt Datasheet - Page 13

• Manufacturer Part Number: sc202amltrt
• Description: 3.5mhz, 500ma Step-down Regulator With Integrated Inductor And Digital Programmable Output
• Manufacturer: Semtech Corporation

Applications Information (continued)
The resistor values can be determined using the following
equation.
where V
setting selected by the CTL pins, R
between the output capacitor and the SNS pin, R
resistor between the SNS pin and ground, and I
leakage current into the SNS pin during normal operation.
The current into the SNS pin is typically 1µA, so the last
term of the equation can be neglected if the current
through R
value of 10kΩ or lower will simplify the design. If I
neglected and R
the following equation.
Inserting resistance in the feedback loop will adversely
affect the system’s transient performance if feed-forward
capacitance is not included in the circuit. The circuit in
Figure 1 illustrates how the resistor divider and feed-
forward capacitor can be added to the SC202A
schematic.
The value of feed-forward capacitance needed can be
determined using the following equation.
To simplify the design, it is recommended to program the
output setting to 1.0V, use resistor values smaller than
V
Figure 1 – Application Circuit with External Resistors
IN
C
R
V
Enable
FF
OUT
FB
1
OUT
4
R
V
FB2
is the desired output voltage, V
SET
FB
10
is much larger than 1µA. Selecting a resistor
C
2
IN
6
V
R
FB2
OUT
FB
R
R
1
is fixed, R
V
IN
CTL3
CTL2
CTL1
CTL0
FB
FB
SET
1
R
2
V
SC202A
V
FB
V
SET
OUT
SET
2
V
FB1
I
SNS
V
OUT
GND
OUT
SNS
SET
can be determined using
R
0
V
FB
5 .
SET
1
R
FB1
2
FB2
C
OUT
C
R
SET
0
is the resistor
FF
FB1
5 .
is the voltage
SNS
FB2
is the
is the
V
SNS
OUT
is
10kΩ, and include a feed-forward capacitance calculated
with the previous equation. If the output voltage is set to
1.0V, the previous equation reduces to the following
equation.
Example:
An output voltage of 1.3V is desired, but this is not a pro-
grammable option. What external component values for
Figure 1 are needed?
Solution: To keep the circuit simple, set R
current into the SNS pin can be neglected and set the
CTL3-0 pins to 0010 (1.0V setting). The necessary compo-
nent values for this situation are shown by the following
equations.
PWM Operation
Normal PWM operation occurs when the output load
current exceeds the PSAVE threshold. In this mode, the
PMOS high side switch is activated with the duty cycle
required to produce the output voltage programmed by
the CTL pins. An internal synchronous NMOS rectifier
eliminates the need for an external Schottky diode on
the LX pin. The duty cycle (percentage of time PMOS is
active) increases as V
voltage regulation. As the input voltage approaches the
programmed output voltage, the duty cycle approaches
100% (PMOS always on) and the device enters a pass-
through mode until the input voltage increases or the
load decreases enough to allow PWM switching to
resume.
Power Save Mode Operation
When the load current decreases below the PSAVE
threshold, PWM switching stops and the device auto-
matically enters PSAVE mode. This threshold varies
depending on the input voltage and output voltage
C
C
R
FF
FF
FB
1
8
8
R
10
FB
10
2
6
6
V
R
OUT
R
V
FB
V
V
OUT
FB
1
SET
OUT
1
IN
V
V
V
OUT
SET
decreases to maintain output
OUT
0
0
5 .
5 .
1
2
k 3
1
2
. 5
69
nF
FB2
SC202A
to 10kΩ so
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