sc172mltrt Semtech Corporation, sc172mltrt Datasheet - Page 11

• Manufacturer Part Number: sc172mltrt
• Description: 2a Ecospeed Synchronous Step-down Regulator With Optional Ultrasonic Power Save
• Manufacturer: Semtech Corporation

Q2
Q1
Applications Information
SC172 Synchronous Buck Converter
The SC172 is a step down synchronous buck dc-dc regu-
lator. The SC172 is capable of 2A 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 SC172 can operate with an input voltage ranging
from 3V to 5.5V.
Psuedo-fixed Frequency Adaptive On-time Control
The PWM control method used by the SC172 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.
© 2010 Semtech Corporation
Figure 1 — PWM Control Method, V
V
V
LX
IN
C
C
L
OUT
ESR
IN
T
V
ON
FB
+
V
OUT
LX
V
Ripple
OUT
FB
FB threshold
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
and inversely proportional to input voltage. With this
adaptive on-time arrangement, the device automatically
anticipates the on-time needed to regulate V
present V
The advantages of adaptive on-time control are:
One-Shot Timer and Operating Frequency
The one-shot timer operates as shown in Figure 2. The
FB Comparator output goes high when V
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
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
voltage reaches V
high-side MOSFET turns off.
This method automatically produces an on-time that is
proportional to V
Under steady-state operation conditions, the switching
frequency can be determined from the on-time by the
following equation.
OUT
•
•
•
•
•
and V
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
IN
IN
condition and at the selected frequency.
; the period is proportional to output voltage
OUT
OUT
, the on-time is completed and the
OUT
f
SW
and inversely proportional to V
from LX. One comparator input
=
T
ON
V
OUT
×
IN
V
. When the capacitor
IN
FB
is less than
OUT
for the
11
IN
.