SI9176_04 Datasheet, PDF (5/17 Pages) Vishay Siliconix – High Performance Step-Down DC-DC Converter With Adjustable Output Voltage

FUNCTIONAL BLOCK DIAGRAM
V_DD
Si9176
Si9176
Vishay Siliconix
V_IN
PMOS
Current Sense
FB
REF
Error
Amplifier
PWM
−
+
Clamp
Comparator
−
PWM and
Pulse
+
Skipping
L_X
Logic
To IC Bias
Voltage
Reference,
UVLO
2 MHz
Oscillator
AGND AGND
MODE SD
PGND
DETAIL DESCRIPTION
General
The Si9176 is a high efficiency synchronous dc-dc converter
that is ideally suited for lithium ion battery or three cell alkaline
applications, as well as step-down of 3.3-V or 5.0-V supplies.
The major blocks of the Si9176 are shown in the Functional
Block Diagram. The 0.25-W internal MOSFETs switching at a
frequency of 2-MHz minimize PC board space while providing
high conversion efficiency and performance. The high
frequency error-amplifier with built-in loop compensation
minimizes external components and provides rapid output
settling times of <30 ms. Sensing of the inductor current for
control is accomplished internally without power wasting
resistors.
Start-Up
When voltage is applied to V_IN and V_DD, the under-voltage
lockout (UVLO) circuit prevents the oscillator and control
circuitry from turning on until the voltage on the exceeds 2.4 V.
With a typical UVLO hysteresis of 0.1 V, the converter
operates continuously until the voltage on V_IN drops below
2.3 V, whereupon the converter shuts down. This hysteresis
prevents false start-stop cycling as the input voltage
approaches the UVLO switching threshold. Start-up is always
accomplished in PWM mode to ensure start-up under all load
Document Number: 71753
S-41148—Rev. C, 14-Jun-04
conditions. Switching to other modes of operation occurs
according to the state of the MODE pin and the load current.
The start-up sequence occurs after SD switches from LOW to
HIGH with V_IN applied, or after V_IN rises above the UVLO
threshold and SD is a logic HIGH.
Mode Control (MODE)
The MODE pin allows the user to control the mode of operation
or to enable the Si9176 to automatically optimize the mode of
operation according to load current. There are three different
modes of operation as controlled by the MODE pin. Switching
waveforms are shown in the Typical Switching Waveform
sections, page 9.
PWM Mode (MODE pin = HIGH)
With the MODE pin in the logic HIGH condition, the Si9176
operates as a 2-MHz fixed frequency voltage mode converter.
A NMOS synchronous rectification MOSFET transistor
provides very high conversion efficiency for large load currents
by minimizing the conduction losses. PWM mode provides low
output ripple, fast transient response, and switching frequency
synchronization. Output load currents can range from 0 to
600 mA.
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