Vishay Siliconix
SiC417
Document Number: 69062
S10-1367-Rev. D, 14-Jun-10
www.vishay.com
7
ELECTRICAL CHARACTERISTICS
APPLICATIONS INFORMATION
SiC417 Synchronous Buck Converter
The SiC417 is a step down synchronous buck dc-to-dc
converter with integrated power FETs and programmable
LDO. The SiC417 is capable of 10 A operation at very high
efficiency in a tiny 5 mm x 5 mm - 32 pin package. The
programmable operating frequency range of 200 kHz to
1 MHz, enables the user to optimize the solution for minimum
board space and optimum efficiency.
The buck controller employs pseudo-fixed frequency
adaptive on-time control. This control scheme allows fast
transient response thereby lowering the size of the power
components used in the system.
Input Voltage Range
The SiC417 requires two input supplies for normal operation:
V
IN
and V5V. V
IN
operates over the wide range from 3 V to
28 V. V5V requires a 5 V supply input that can be an external
source or the internal LDO configured to supply 5 V. When
V
IN
is less than ~ 6 V then an external 5 V supply must be
tied to V5V.
Pseudo-Fixed Frequency Adaptive On-Time Control
The PWM control method used for the SiC417 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.
The adaptive on-time is determined by an internal oneshot
timer. When the one-shot is triggered by the output ripple, the
device sends a single on-time pulse to the highside
MOSFET. The pulse period is determined by V
OUT
and V
IN
;
the period is proportional to output voltage and inversely
proportional to input voltage. With this adaptive on-time
arrangement, the device automatically anticipates the
on-time needed to regulate V
OUT
for the present V
IN
condition and at the selected frequency.
The adaptive on-time control has significant advantages over
traditional control methods used in the controllers today.
" Reduced component count by eliminating DCR sense or
current sense resistor as no need of a sensing inductor
current.
" Reduced Saves external components used for
compensation by eliminating the no error amplifier and
other components.
" Ultra fast transient response because of fast loop,
absence of error amplifier speeds up the transient
response.
" Predictable frequency spread because of constant on-time
architecture.
" Fast transient response enables operation with minimum
output capacitance
Overall, superior performance compared to fixed frequency
architectures.
Over Current Protection: V
IN
= 12 V, V
OUT
= 1.2 V
Ultra-Sonic Power-Save at I
OUT
= 0 A
Figure 1 - Output Ripple and PWM Control Method
V
IN
C
IN
V
LX
Q1
Q2
L
ESR
+
FB
V
LX
t
ON
V
FB
C
OUT
V
OUT
FB threshold
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