A8450
Automotive Multioutput Voltage Regulator
Functional Description
Buck Converter with Switching Regulator.
A cur-
rent-mode, variable frequency buck dc-to-dc converter and
switching regulator are integrated in the A8450, as shown in
figure 2. This feature allows the device to efficiently handle
power over a wide range of input supply levels. The dc-to-dc
converter outputs 5.7 V typical, and has an overcurrent limit
of 1.2 A typical.
The converter employs a soft-start feature. This ramps the
converter output voltage and limits the maximum demand on
V
REG
by controlling the inrush current required at power-on
to charge the external capacitor, COUT, and any dc load.
An internal charge pump provides gate drive for the
N-channel MOSFET buck switch. A 100% duty cycle is
implemented when using low V
BB
input voltages.
At V
BB
lower than 12 V, off-time, t
OFF
, is reduced, as shown
in figure 3. This reduction keeps the switching frequency,
f
PWM
, within a reasonable range and lowers the ripple cur-
rent. Lowering the ripple current at low V
BB
levels prevents
degradation of linear regulator headroom due to V
REG
ripple
voltage.
5 V Linear Regulators.
Two 5 V medium-power linear regula-
tors are provided. These low-dropout regulators feature foldback
current limiting for short-to-supply protection. When a direct
VBB
Buck Converter
Buck Switch
VCP
Switching
Regulator
Control
t
OFF
LX
t
OFF
by V
BB
5.5
5.0
4.5
4.0
3.5
4.75 µs
12 V
t
OFF
(µs)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
5
7
9
11
13
15
0.58 µs
6.02 V
2.05 µs
11 V
V
BB
(V)
Figure 3. When VBB falls below 12 V, t
OFF
decreases
L1
100 µH
D1
COUT
100 µF
ENB
Clock
Counter
1.2 A Limit
Soft Start
Ramp
Generation
Error
Bandgap
1.22 V
I
DEMAND
I
PEAK
Compensation
Clamp
VREG
Figure 2. Buck converter with switching regulator
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
A8450KLB-DS, Rev. 1
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
