SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 18

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

is no voltage at the switching regulator output or current
in the PGOOD pull-up resistor. If the PGOOD output high
level (= V
up from the input or a separate power supply will be the
only choice.
Sequencing the Outputs
As mentioned above, pulling either soft-start pin low with
an external transistor shuts off the corresponding
regulator (Figure 11). Releasing the soft-start pin enables
that channel and allows it to start. Delaying the release
of the soft-start pin of one channel with respect to the
other is a straightforward way of sequencing the outputs.
Figure 11(a) shows this method using two external
transistors M
channel 1 to start. Channel 2 is then enabled after time
T
The PGOOD output of one channel can also be used in
conjunction with the soft-start pin of the other channel
to delay start of that regulator. This method is depicted
in Figure 11(b). SS2 is pulled low and channel 2 is kept
off until channel 1 output rises to 90% of its set voltage.
A drawback of this approach is that only PGOOD2 is
available as a logic output.
Loop Compensation
Figure 12 shows a simplified equivalent circuit of a step-
down converter. The power stage, which consists of the
POWER MANAGEMENT
Applications Information
D
2005 Semtech Corp.
.
OUT
) is unacceptably low, then power good pull-
1
and M
2
. M
1
is turned off first, allowing
C6
Figure 12. Simplified Control Loop Equivalent Circuit
V
IN
V
COMP
R5
C5
RO
GMA =
280
GMP = 5.7
POWER
STAGE
-1
REFERENCE
-
+
VOLTAGE
-1
FB
1V
18
current-mode PWM comparator, the power switch, the
freewheeling diode and the inductor, feeds the output
network. The power stage can be modeled as a voltage-
controlled current source, producing an output current
proportional to its controlling input V
transconductance G
closed, the control-to-output transfer function
a dominant-pole p
than that of the output filter pole.
where C
load resistance and n (depending on duty ratio, slope
compensation, frequency and passive components) is
usually between 1 and 2.
If C
situates well beyond half the switching frequency. The
low frequency gain of the control-to-output transfer
function is simply the product of power stage
transconductance and the equivalent load resistance
(Figure 13).
The transfer functions of the feedback network and the
error amplifier are:
C11
1
is ceramic, then its ESR zero can be neglected as it
p
2
R1
R2
1
is the output capacitor, R
V
nI
OUT
ESR
I OUT
C1
OUT
C
1
2
R OUT
located at a frequency slightly higher
MP
V OUT
R
OUT
is 5.7
n
C
1
-1
. With the current loop
OUT
is the equivalent
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SC2440
COMP
v
v
COMP
OUT
.
has
Its
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