A4408 Datasheet, PDF (19/54 Pages) Allegro MicroSystems – PWM frequency can be synchronized to external clock

A4408
Adjustable Frequency Buck or Buck-Boost Pre-Regulator with a
Synchronous Buck, 3 Internal LDOs, Watchdog Timer, NPOR, and FF0/FF1
FUNCTIONAL DESCRIPTION
Overview
The A4408 is a power management IC designed for automotive
applications. It contains a pre-regulator plus four DC post-
regulators to create the voltages necessary for typical automotive
applications such as electrical power steering and automatic
transmission control.
The pre-regulator can be configured as a buck or buck-boost
regulator. Buck-boost is required for applications that must work
at extremely low battery voltages. This pre-regulator generates
a fixed 5.35 V and can deliver up to 1 A to power the internal
or external post-regulators. These post-regulators generate the
various voltage levels for the end system.
The A4408 includes four internal post-regulators: three linear
regulators and one adjustable output synchronous buck regulator.
The synchronous buck regulator was designed to deliver
1.25 V / 700 mA but will produce higher voltages if a feedback
resistor divider is used.
Buck-Boost Pre-Regulator (VREG)
The pre-regulator incorporates an internal high-side buck switch
and a boost switch gate driver. An external freewheeling Schottky
diode and an LC filter are required to complete the buck con-
verter. By adding a MOSFET and a Schottky diode, the boost
configuration can maintain all outputs with input voltages as low
as 2.8 V. The A4408 includes a compensation pin (COMP1) and a
soft-start pin (SS1) for the pre-regulator.
The A4408 can maintain its outputs over a wide range of input
voltages and slew rates. Actual boost performance is shown in
Figure 5 and Figure 6 with voltages swinging between 2.9 and
18 V, and V_VIN slew rates ranging from 0.3 to 100V/ms.
The buck-boost pre-regulator provides protection and diagnostic
functions.
1. Overvoltage protection
2. High voltage rating for load dump
3. Switch-node-to-ground short-circuit protection
4. Open freewheeling diode protection
5. Pulse-by-pulse current limit
6. Hiccup short circuit protection – lab measurement shown in
Figure 7 and detailed timing diagram shown in Figure 5
Figure 5: A4408 Buck-Boost operation at full load
V_VIN slew rates ranging from 0.3 V/ms to 1.6 V/ms
Typical of an automotive START/STOP waveform
V_VIN(TYP) = 12 V, V_VIN(MIN) = 2.9 V, 10 ms/DIV
CH1=VIN, CH2=VREG, CH3=V5, CH4=3V3, M1=1V25, M2=V5P
Figure 6: A4408 Buck-Boost operation at full load
V_VIN slew rates of 100 V/ms ‒
V5P deviates less than 0.2%
V_VIN(TYP) = 12 V, V_VIN(MIN) = 4 V, V_VIN(MAX) = 18 V
CH1=VIN, CH2=VREG, CH3=V5, CH4=V5P, 500 µs/DIV
Allegro MicroSystems, LLC
19
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com