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参数资料
| 型号： | APEK4403GEU-01-T-DK |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 5/16页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR A4403 |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，非隔离 |
| 输入电压： | 9 V ~ 46 V |
| 稳压器拓扑结构： | 降压 |
| 板类型： | 完全填充 |
| 已供物品： | 板，CD |
| 已用 IC / 零件： | A4403 |
| 相关产品： | 620-1273-2-ND - IC REG BUCK ADJ 3A 16QFN |
| 其它名称： | 620-1387 |
A4403
Valley Current Mode Control Buck Converter
Functional Description
? V OUT
– 1 ? ? ,
?
? V FB
Basic Operation TheA4403 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 IN .
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 m Ω by two 100 m Ω
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.
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:
?
R 5 = R 6 (1)
? ?
where (refering to the Functional Block diagram):
R 6 has a value between 750 Ω and 12 k Ω (R6 connected between
the GND and FB pins),
R 5 is the dependent value (R5 connected between the output rail
and the FB pin),
V OUT is the user-configured output regulator voltage, and
V FB is the reference voltage.
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.
Inductor current operating at maximum load
Current Limit level
Maximum load
Constant On-Time
Constant period
Time
Inductor current operating in a “soft” overload
Overload
Current Limit level
Constant On-Time
Extended period
Time
Figure 1. Current limiting during overload
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
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