a4403geu-t Allegro MicroSystems, Inc., a4403geu-t Datasheet - Page 5

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

A4403
Basic Operation The A4403 is a buck converter that utilizes
valley current-mode control. The on-time is set by the amount
of current that flows into the TON pin. This is determined by the
value of the TON resistors chosen (R1 and R2 in the Functional
Block diagram) and the magnitude of the input voltage, V
Under a specific set of conditions, an on-time can be set that
then dictates the switching frequency. This switching frequency
remains reasonably constant throughout load and line conditions
as the on-time varies inversely with the input voltage. The Switch
On-Time and Switching Frequency section provides more details
on this subject.
At the beginning of the switching cycle, the buck switch is turned
on for a fixed period that is determined by the current flowing
into TON. Once the current comparator trips, a one-shot mono-
stable, the On Timer, is reset, turning off the switch. The current
through the inductor then decays. This current is sensed through
the external sense resistors (R3 and R4), and then compared
against the current-demand signal. The current-demand signal is
generated by comparing the output voltage against an accurate
bandgap reference. After the current through the sense resistors
decreases to the valley of the current-demand signal, the On
Timer is set to turn the buck switch back on again and the cycle is
repeated.
Under light load conditions, the converter automatically operates
in pulse frequency modulation (PFM) mode to maintain regu-
lation. This mode of operation ensures optimum efficiency as
switching losses are reduced.
Overcurrent Protection The converter utilizes pulse-by-pulse
valley current limiting, which operates when the current through
the sense resistors, R3 and R4 (set for 50 mΩ by two 100 mΩ
resistors in parallel), increases above 3.6 A typical at the valley
point. The corresponding sense voltage (at the ISEN pin) that cre-
ates a current limiting condition is 180 mV typical. It is possible,
by careful selection of the sense resistors, to reduce the current
limit for systems with maximum loads of less than 3 A.
During an overload condition, the switch is turned on for the
period determined by the constant on-time circuitry. The switch
off-time is extended until the current decays to the current limit
value of 3.6 A typical (which corresponds to a sense voltage of
180 mV). The switch is then turned on again.
Because no slope compensation is required in this control
scheme, the current limit is maintained at a reasonably constant
level across the input voltage range.
Functional Description
Valley Current Mode Control Buck Converter
IN
.
Figure 1 illustrates how the current is limited during an overload
condition. The current decay (period with switch off) is propor-
tional to the output voltage. As the overload is increased, the out-
put voltage tends to decrease and the switching period increases.
Output Voltage Selection The output voltage of the converter
is set by selecting the appropriate feedback resistors, using the
following formula:
where (refering to the Functional Block diagram):
R
R
V
V
The tolerance of the feedback resistors influences the voltage set-
point. It is therefore important to consider the tolerance selection
when targeting an overall regulation figure.
Figure 1. Current limiting during overload
GND and FB pins),
and the FB pin),
6
5
OUT
FB
has a value between 1 and 12 kΩ (R6 connected between the
is the dependent value (R5 connected between the output rail
is the reference voltage.
is the user-configured output regulator voltage, and
Inductor current operating at maximum load
Inductor current operating in a “soft” overload
Constant period
Constant On-Time
Constant On-Time
R
5
=
R
6
Extended period
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
⎛
⎜
⎜
⎝
V
Time
Time
V
OUT
Current Limit level
FB
Current Limit level
– 1
⎞
⎟
⎟
⎠
,
Overload
Maximum load
(1)
5