Wide Input Voltage 3.0 A Step Down Regulator
A8498
5
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION
The A8498 is a fixed off-time, current-mode–controlled buck
switching regulator. The regulator requires an external clamping
diode, inductor, and filter capacitor, and operates in both continu-
ous and discontinuous modes. An internal blanking circuit is used
to filter out transients resulting from the reverse recovery of the
external clamp diode. Typical blanking time is 200 ns.
The value of a resistor between the TSET pin and ground deter-
mines the fixed off-time (see graph in the t
OFF
section).
V
OUT
. The output voltage is adjustable from 0.8 to 24 V, based on
the combination of the value of the external resistor divider and
the internal 0.8 V ±2% reference. The voltage can be calculated
with the following formula:
V
OUT
= V
FB
× (1 + R1/R2) (1)
Light Load Regulation
To maintain voltage regulation during light load conditions, the
switching regulator enters a cycle-skipping mode. As the output
current decreases, there remains some energy that is stored during
the power switch minimum on-time. In order to prevent the out-
put voltage from rising, the regulator skips cycles once it reaches
the minimum on-time, effectively making the off-time larger.
Soft Start
An internal ramp generator and counter allow the output to
slowly ramp up. This limits the maximum demand on the external
power supply by controlling the inrush current required to charge
the external capacitor and any dc load at startup. Internally, the
ramp is set to 10 ms nominal rise time. During soft start, current
limit is 3.5 A minimum.
The following conditions are required to trigger a soft start:
• V
IN
> 6 V
• ENB pin input falling edge
• Reset of a TSD (thermal shut down) event
V
BIAS
To improve overall system efficiency, the regulator output, V
OUT
,
is connected to the VBIAS input to supply the operating bias cur-
rent during normal operating conditions. During startup the cir-
cuitry is run off of the VIN supply. VBIAS should be connected
to VOUT when the V
OUT
target level is between 3.3 and 5 V. If
the output voltage is less than 3.3 V, then the A8498 can operate
with an internal supply and pay a penalty in efficiency, as the bias
current will come from the high voltage supply, VIN. VBIAS can
also be supplied with an external voltage source. No power-up
sequencing is required for normal operation.
ON/OFF Control
The ENB pin is externally pulled to ground to enable the device
and begin the soft start sequence. When the ENB is open cir-
cuited, the switcher is disabled and the output decays to 0 V.
Protection
The buck switch will be disabled under one or more of the fol-
lowing fault conditions:
• V
IN
< 6 V
• ENB pin = open circuit
• TSD fault
When the device comes out of a TSD fault, it will go into a soft
start to limit inrush current.
t
OFF
The value of a resistor between the TSET pin and ground deter-
mines the fixed off-time. The formula to calculate t
OFF
(µs) is:
,
t
OFF
R
TSET
=
10.2 × 10
9
BIAS
(2)
where R
TSET
(kΩ) is the value of the resistor. Results are shown
in the following graph:
0
20
40
60
80
100
120
140
160
180
200
12 3 45678 910111213141516
Off-Time Setting versus Resistor Value
R
TSET
(kΩ)
t
OFF
(µs)
V
BIAS
= 5 V
V
BIAS
= 3.3 V
