LA5724MC 데이터 시트보기 (PDF) - SANYO -> Panasonic
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LA5724MC Datasheet PDF : 6 Pages
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LA5724MC
Part selection and set
1. Resistors R1 and R2
R1 and R2 are resistors to set the output voltage. When the large resistance value is set, the error of set voltage increases
due to the VOS pin current. The output voltage may also increases due to the leak current of switching transistor at light
load. In consequence, it is essential to see R1 and R2 currents to around 500μA.
R1
=
1.23V
500μA
≈
2.4kΩ
2.0kΩ to 2.4kΩ recommended
R2
=
VOUT
( 1.23V
-
1)
×
R1
The calculation equation gives the output voltage set by R1 and R2.
VOUT
=
(1
+
R2
R1)
×
1.23V(typ)
2. Capacitor C1, C2 and C3
The large ripple current flows through C1 and C2, so that the high-frequency low-impedance product for switching
power supply must be used. Do not use, for C2, a capacitor eith extremely small equivalent series resistance (ESR),
such as ceramic capacitor, tantalum capacitor. Otherwise, the output waveform may develop abnormal oscillation. The
C2 capacitance and ESR value stabilization conditions are as follows:
1
2×π×C2×ESR
≤
20kHz
C3 is a capacitor for phase compensation of the feedback loop. Abnomal oscillation may occur when the C2
capacitance value is small or the equivalent series resistance is small. In this case, addition of the capacitance of C3
enable phase compensation, contributing to stabilization of power supply.
3. Input capacitor: Effecitive-value current
The AC ripple current flowing in the input capacitor is larger than that in the output capacitor. The equation expressing
the effective-value current is as folloes. Use the capacitor wiyhin the reted current range.
IC1 = Vout (Iout2 (1− Vout ) + 1 × ΔIR 2 [Arms]
Vin
Vin 12
4. Output capacitor: Effective-value current
The AC ripple current flowing in the capacitor is the triangular wave. Therefore, its effective value is obtained from the
following equation. Select the output capacitor so that it does not exceed the allowable ripple current value.
IC2 =
1
23
×
VOUT(VIN-VOUT)
L×fSW×VIN
[Arms]
fSW = switching frequency ··· 160kHz
5. Choke coil
Note that choke coil heating due to overload or load shorting may be a problem. The inductance valuecan be
determined from the following equation once the input voltage, output voltage, and current ripple conditions are known.
ΔIR indicates the ripple current value.
Reference example: VIN = 12V, VOUT = 5V, ΔIR = 150mA
L
=
VIN
-
VOUT
ΔIR
-
Vsat
×
Tom
=
12
-
5.0 -
0.15
0.4
×
2.8×10-6
≈ 120µH
T
Ton = (VIN - VOUT - Vsat) / (VOUT + VF)+1)
Toff = T - Ton
T: Switching repetition period ··· 6.25µs is assumed for the calculation
VF: Schottky diode forward voltage ··· 0.4V is assumed for the calculation
No.A2021-5/6
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