SC417EVB Ver la hoja de datos (PDF) - Semtech Corporation
Número de pieza
componentes Descripción
Fabricante
SC417EVB Datasheet PDF : 29 Pages
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SC417/SC427
Applications Information
Synchronous Buck Converter
The SC417/SC427 is a step down synchronous DC-DC buck
converter with integrated power MOSFETs and a program-
mable 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 range of
200kHz 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 SC417/SC427 requires two input supplies for normal
operation: VIN and V5V. VIN operates over the wide range
from 3V to 28V. V5V requires a 5V supply input that can be
an external source or the internal LDO configured to
supply 5V.
Psuedo-fixed Frequency Adaptive On-time Control
The PWM control method used by the SC417/SC427 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
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 and
OUT
V ; the period is proportional to output voltage and
IN
inversely proportional to input voltage. With this adaptive
on-time arrangement, the device automatically antici-
pates the on-time needed to regulate V for the present
OUT
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
One-Shot Timer and Operating Frequency
The one-shot timer operates as shown in Figure 2. The FB
Comparator output goes high when V is less than the
FB
internal 500mV reference. This feeds into the gate drive
and turns on the high-side MOSFET, and also starts the
one-shot timer. The one-shot timer uses an internal com-
parator and a capacitor. One comparator input is con-
nected to V , the other input is connected to the
OUT
capacitor. When the on-time begins, the internal capaci-
tor charges from zero volts through a current which is
proportional to V . When the capacitor voltage reaches
IN
V , the on-time is completed and the high-side MOSFET
OUT
turns off.
Figure 1 — PWM Control Method, V Ripple
OUT
13
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