SC403MLTRT Semtech, SC403MLTRT Datasheet - Page 15

• Manufacturer Part Number: SC403MLTRT
• Manufacturer: Semtech

Specifications of SC403MLTRT

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Applications Information
Synchronous Buck Converter
The SC403 is a step down synchronous DC-DC buck con-
verter with integrated power MOSFETs and a 200mA pro-
grammable LDO. The device operates at a current up to
6A at very high efficiency. A space saving 5x5 (mm) 32-pin
package is used. The programmable operating frequency
of up to 1MHz enables optimizing the configuration for
PCB area and efficiency.
The buck controller uses a pseudo-fixed frequency adap-
tive on-time control. This control method allows fast tran-
sient response which permits the use of smaller output
capacitors.
Input Voltage Requirements
The SC403 requires two input supplies for normal opera-
tion: V
28V. VDD requires a supply voltage between 3V to 5V that
can be an external source or the internal LDO from V
Power Up Sequence
The SC403 initiates a start up when VIN, VDD, and EN/PSV
pins are above the applicable thresholds. When using an
external bias supply for the V
that the V
voltage is present because V
time. A 10Ω resistor must be placed between the external
V
during power-up and or shutdown situations where V
could exceed V
Shutdown
The SC403 can be shutdown by pulling either VDD or EN/
PSV pin below its threshold. When using an external
supply voltage for V
while the V
be placed between the external V
pin to avoid damage to the device.
When the VDD pin is active and EN/PSV is at low logic
level, the output voltage discharges through an internal
FET.
Psuedo-fixed Frequency Adaptive On-time Control
The PWM control method used by the SC403 is pseudo-
fixed frequency, adaptive on-time, as shown in Figure 1.
DD
supply and the VDD pin to avoid damage to the device
IN
and V
DD
IN
is applied to the device only after the V
voltage is still present. A 10Ω resistor must
DD
. V
IN
unexpectedly.
IN
operates over a wide range from 3V to
DD
, the VDD pin must be deactivated
DD
DD
voltage, it is recommended
cannot exceed V
DD
supply and the VDD
IN
at any
IN
.
DD
IN
The ripple voltage generated at the output capacitor ESR
is used as a PWM ramp signal. This ripple is used to trigger
the controller on-time.
The adaptive on-time is determined by an internal one-
shot timer. When the one-shot is triggered by the output
ripple, the device sends a single on-time pulse to the high-
side MOSFET. The pulse period is determined by V
V
inversely proportional to input voltage. With this adaptive
on-time arrangement, the device automatically antici-
pates the on-time needed to regulate V
V
The advantages of adaptive on-time control are:
IN
IN
•
•
•
•
•
. The period is proportional to output voltage and
Q1
Q2
condition and at the selected frequency.
V
IN
Figure 1 — PWM Control Method, V
Predictable operating frequency compared to
other variable frequency methods.
Reduced component count by eliminating the
error amplifier and compensation components.
Reduced component count by removing the
need to sense and control inductor current.
Fast transient response — the response time is
controlled by a fast comparator instead of a typi-
cally slow error amplifier.
Reduced output capacitance due to fast tran-
sient response
V
LX
C
IN
L
TON
V
+
ESR
FB
C
OUT
V
LX
OUT
FB Threshold
OUT
for the present
Ripple
SC403
FB
V
OUT
OUT
and
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