a4403geu-t Allegro MicroSystems, Inc., a4403geu-t Datasheet - Page 6

• Manufacturer Part Number: a4403geu-t
• Description: Valley Current Mode Control Buck Converter
• Manufacturer: Allegro MicroSystems, Inc.

A4403
In general, the feedback resistors should have the lowest resis-
tance possible, to minimize any noise pick-up effects and to
minimize voltage offsets on the output caused by the bias current,
I
resistances does introduce another loading effect on the output,
which has an effect on the standby current.
It should be noted that a minimum load of 1 mA is required (see
the Light Load Operation section). This may be provided by the
feedback resistors.
Disable The converter is enabled by pulling the DIS pin low.
Once enabled, the output converter is started-up under the control
of the soft-start routine.
To disable the converter, the DIS pin can simply be disconnected
(open circuit).
Soft Start A soft-start routine is initiated when: DIS = 0, no
thermal shutdown exists, and V
supplies are above the minimum values. Note that an overcurrent
event does not initiate a soft start, unless the converter is recover-
ing from a thermal shutdown condition.
The soft-start routine controls the rate of rise of the reference
voltage, which in turn controls the output voltage. This function
minimizes the amount of inrush current drawn from VIN and
potential voltage overshoot on the output rail, VOUT.
The soft-start period, T
that charges the external capacitor (C5) connected to the SS pin.
Control by the soft-start routine is completed when the SS pin
reaches 0.8 V. The duration of T
priate capacitance, according to the formula:
Note: If the soft start function is not required for the application,
a 220 kΩ resistor should be connected between the SS pin and
GND. Without soft start, or with a soft start period that is too
rapid, coupled with a high load that is present during start-up, the
converter may operate in current limit, placing maximum stress
on the input circuit.
Assuming no load is drawn until the start-up process is complete,
the current drawn from the input supply is determined by how
quickly the output capacitors (C3 and C4) are charged. The out-
put capacitors are charged according to the following formula:
BIAS
, flowing out of the FB node into R6. Reducing the feedback
t
CHARGE
T
SS
SS
=
=
, is set by an internal current source
10 ×10
C
5
C
× 0.8
OUT
IN
SS
and the internal housekeeping
–6
is set by selecting the appro-
× V
I
VIN
OUT
.
,
Valley Current Mode Control Buck Converter
(2)
(3)
where I
For example, if you limited I
5 V and C
This means a soft-start duration greater than 400 μs should be
selected to ensure the inrush current is less than 250 mA.
Shutdown The converter is disabled in the event of either an
overtemperature event, or an undervoltage on VIN (V
on an internal housekeeping supply.
As soon as any of the above faults have been removed and
assuming DIS = 0, the output voltage, V
the control of the soft-start routine.
Output Overvoltage Protection In the event of an over-
voltage condition appearing on the output rail, the FB terminal
will also experience the overvoltage, scaled by the feedback resis-
tors. If the FB terminal voltage rises above the nominal voltage
by 10% (typical), the on-time of the buck switch will terminate
and the switch will remain off until the FB voltage reduces to the
correct V
Switch On-Time and Switching Frequency The switch
on-time effectively determines the operating frequency of the
converter. The selection of the operating frequency is generally
a trade-off between the size of the external passive components
(inductor, and input and output capacitors) and switching losses.
Another consideration in selecting the switching frequency is to
ensure that none of the on- or off-time limits are reached under
extreme conditions.
The minimum on-time occurs at maximum input voltage and
minimum load. Consider the following example.
Given:
V
where V
sense resistors (R3 and R4). Then, the minimum on-time is:
IN
(max) = 46 V, V
VIN
f
FB
is the voltage drop of the recirculation diode (D1) and
T
OUT
T
is the input supply current.
on
on
range.
(min)
(min) =
= 20 μF, the soft start time could be determined as:
t
CHARGE
=
OUT
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
46 + 0.5
V
5 + 0.5
= 5 V, f
V
=
OUT
IN
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
20 μF × 5 V
VIN
+ V
+ V
0.25 A
SW
to 250 mA, and assumed V
f
⎞
⎟
⎟
⎠
f
×
⎞
⎟
⎟
⎠
= 1 MHz, and:
×
1× 10
f
OUT
1
SW
1
=
6
400 μs
, is brought-up under
=
118 ns
,
INUVR
) or
OUT
(4)
=
6