SC402B View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC402B' PDF : 32 Pages View PDF
SC402B
Applications Information
Synchronous Buck Converter
The SC402B is a step down synchronous DC-DC buck
converter with integrated power MOSFETs and a 200mA
capable programmable LDO. The device is capable of 10A
operation 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 configu-
ration 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.
In addition to the following information, the user can
click on the applicable link to go to the SC402B online
C-SIM design and simulation tool, which will lead the
user through the design process.
Input Voltage Requirements
The SC402B requires two input supplies for normal opera-
tion: VIN and VDD. VIN operates over a wide range from 3V
to 28V. VDD requires a 3V to 5.5V supply input that can be
an external source or the internal LDO configured to
supply 3V to 5.5V from VIN.
Power Up Sequence
When the SC402B uses an external power source at the
VDD pin, the switching regulator initiates the start up
when VIN, VDD and EN/PSV are above their respective
thresholds. When EN/PSV is at a logic high, VDD needs to
be applied after VIN rises. It is also recommended to use
a10Ω resistor between an external power source and the
VDD pin. To start up by using the EN/PSV pin when both
VDD and VIN are above their respective thresholds, apply
EN/PSV to enable the start-up process. For SC402B in self-
biased mode, refer to the LDO section for a full
description.
Shutdown
The SC402B can be shutdown by pulling either VDD or
EN/PSV below its threshold. When using an external
power source, it is recommended that the VDD voltage
ramps down before the VIN voltage. When VDD is active
and EN/PSV at low logic, the output voltage discharges
into the VOUT pin through an internal FET.
Psuedo-fixed Frequency Adaptive On-time Control
The PWM control method used by the SC402B is pseudo-
fixed frequency, adaptive on-time, as shown in Figure 1.
The ripple voltage generated at the output capacitor ESR
is used as a PWM ramp signal. This ripple is used to trigger
the on-time of the controller.
VIN
Q1
VLX
Q2
TON
VLX
CIN
VFB
FB Threshold
L
ESR
+
COUT
VOUT
FB
Figure 1 — PWM Control Method, VOUT Ripple
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 VOUT and
VIN; the period is proportional to output voltage and
inversely proportional to input voltage. With this adaptive
on-time arrangement, the device automatically antici-
pates the on-time needed to regulate VOUT for the present
VIN condition and at the selected frequency.
The advantages of adaptive on-time control are:
• 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
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