LA5724MC

No.A2021-5/6

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 500

μA.

R1 =

1.23V

500

μA ≈ 2.4kΩ

2.0k

Ω 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 =

2

2

IR

12

1

)

Vin

Vout

1

(

Iout

(

Vin

Vout

Δ

×

+

−

[Arms]

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

×

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 - Vsat

ΔIR

× Tom

=

12 - 5.0 - 0.4

0.15

≈ 120µH

Ton =

T

(VIN - VOUT - Vsat) / (VOUT + VF)

+1)

Toff = T - Ton

T: Switching repetition period ··· 6.25

µs is assumed for the calculation

VF: Schottky diode forward voltage ··· 0.4V is

assumed for the calculation