SC171_16 Datasheet, PDF (11/27 Pages) Semtech Corporation – Regulator with Optional Ultrasonic Power Save

SC171
Applications Information
SC171 Synchronous Buck Converter
The SC171 is a step down synchronous buck dc-dc regu-
lator. The SC171 is capable of 1A operation at very high
efficiency in a tiny 3x3-10 pin package. The programma-
ble operating frequency range of 200kHz – 1MHz enables
the user to optimize the solution for minimum board
space and optimum efficiency.
The buck regulator 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 SC171 can operate with an input voltage ranging
from 3V to 5.5V.
Psuedo-fixed Frequency Adaptive On-time Control
The adaptive on-time is determined by an internal one-
shot timer. When the one-shot is triggered by the out-
put ripple, the device sends a single on-time pulse to the
high-side 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 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 compo-
nents.
• 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
typically slow error amplifier.
• Reduced output capacitance due to fast tran-
sient response
The PWM control method used by the SC171 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 trig-
ger the on-time of the controller.
Q1
Q2
V_IN
T_ON
C_IN
V_FB
L
V_LX
ESR
V_LX
FB threshold
V_OUT
One-Shot Timer and Operating Frequency
The one-shot timer operates as shown in Figure 2. The
FB Comparator output goes high when V_FB is less than
the internal 750mV 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
comparator, timing capacitor, and a low pass filter (LPF)
which regenerates V_OUT from LX. One comparator input
is connected to the filtered LX voltage, the other input is
connected to the capacitor. When the on-time begins,
the internal capacitor charges from zero volts through a
current which is proportional to V_IN. When the capacitor
voltage reaches V_OUT, the on-time is completed and the
high-side MOSFET turns off.
C_OUT
+
FB
This method automatically produces an on-time that is
proportional to V_OUT and inversely proportional to V_IN.
Under steady-state operation conditions, the switching
frequency can be determined from the on-time by the
following equation.
Figure 1 — PWM Control Method, V_OUT Ripple
f_SW
=
V_OUT
T_ON × V_IN
© 2010 Semtech Corporation
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