參數(shù)資料
型號(hào)： A8499SLJTR-T
廠商： Allegro MicroSystems, Inc.
英文描述： High Voltage Step-Down Regulator
中文描述： 高電壓降壓穩(wěn)壓器
文件頁數(shù)： 5/7頁
文件大?。?/td> 276K
代理商： A8499SLJTR-T
A8499-DS
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A8499
High Voltage Step-Down Regulator
5
The switching frequency is calculated as follows:
1
=
f
SW
t
ON
+
t
OFF
(4)
Shorted Load.
If the voltage on the FB pin falls below 0.5 V, the
regulator will invoke a 0.8 A typical overcurrent limit to handle
shorted load condition at the regulator output. For low output
voltages at power up and in the case of a shorted output, the off-
time is extended to prevent loss of control of the current limit due
to the minimum on-time of the switcher.
The extension of the off-time is based on the value of the TSET
multiplier and the FB voltage, as shown in the following table:
V
FB
(V)
< 0.25
TSET Multiplier
8 × t
OFF
4 × t
OFF
2 × t
OFF
t
OFF
< 0.50
< 0.75
> 0.75
Component Selection
L1.
The inductor must be rated to handle the total load current.
The value should be chosen to keep the ripple current to a reason-
able value. The ripple current, I
RIPPLE
, can be calculated by:
I
RIPPLE
=
V
L(OFF)
× t
OFF
/
L
(5)
V
L(OFF)
= V
OUT
+ V
f
+
I
L(AVG)
×
R
L
(6)
Example:
Given V
OUT
= 5 V, V
f
= 0.55 V, V
IN
= 42 V, I
LOAD
= 0.5 A, power
inductor with L = 180
μ
H and R
L
= 0.5
Ω
Rdc at 55°C, t
OFF
=
7
μ
s, and R
DS(on)
= 1
Ω
.
Substituting into equation 6:
V
L(OFF)
= 5 V + 0.55 V+ 0.5 A × 0.5
Ω
= 5.8 V
Substituting into equation 5:
I
RIPPLE
= 5.8 V × 7
μ
s / 180
μ
H = 225 mA
The switching frequency, f
SW
, can then be estimated by:
f
SW
= 1 / ( t
ON
+
t
OFF
)
(7)
t
ON
=
I
RIPPLE
×
L / V
L(ON)
(8)
V
L(ON)
= V
IN
–
I
L(AVG)
× R
DS(on)
–
I
L(AVG)
×
R
L
–
V
OUT
(9)
Substituting into equation 9:
V
L(ON)
= 42 V – 0.5 A × 1
Ω
– 0.5 A × 0.5
Ω
– 5 V = 36 V
Substituting into equation 8:
t
ON
= 225 mA × 180
μ
H / 36 V = 1.12
μ
s
Substituting into equation 7:
f
SW
= 1 / (7
μ
s +1.12
μ
s) = 123 kHz
Higher inductor values can be chosen to lower the ripple cur-
rent. This may be an option if it is required to increase the total
maximum current available above that drawn from the switching
regulator. The maximum total current available,
I
LOAD(MAX)
, is:
I
LOAD(MAX)
=
I
CL(MIN)
× t
OFF
/
L
(5)
where I
CL(MIN)
is 2.2 A, from the Electrical Chracteristics table.
D1.
The Schottky catch diode should be rated to handle 1.2 times
the maximum load current. The voltage rating should be higher
than the maximum input voltage expected during all operating
conditions. The duty cycle for high input voltages can be very
close to 100%.
COUT.
The main consideration in selecting an output capacitor
is voltage ripple on the output. For electrolytic output capacitors,
a low-ESR type is recommended.
The peak-to-peak output voltage ripple is simply I
RIPPLE
× ESR.
Note that increasing the inductor value can decrease the ripple
current. The minimum voltage rating of the capacitor is 10 V,
however, because ESR decreases with voltage, the most cost-
effective choice may be rated higher in voltage. The ESR should
be in the range from 50 to 500 m
Ω
.
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