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PDF LA5735MC Data sheet ( Hoja de datos )

Número de pieza LA5735MC
Descripción Separately-Excited Step-Down Switching Regulator
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No Preview Available ! LA5735MC Hoja de datos, Descripción, Manual

Ordering number : ENA2022
LA5735MC
Monolithic Linear IC
Separately-Excited Step-Down
Switching Regulator
(Variable Type)
http://onsemi.com
Overview
The LA5735MC is a separately-excited step-down switching regulator (variable type).
Functions
Time-base generator (300kHz) incorporated.
Current limiter incorporated.
Thermal shutdown circuit incorporated.
Specifications
Absolute Maximum Ratings at Ta = 25°C
Parameter
Symbol
Conditions
Ratings
Unit
Input voltage
SW pin application reverse voltage
VOS pin application voltage
Allowable power dissipation
VIN
VSW
VVOS
Pd max
Mounted on a circuit board.*
34
-1
-0.2 to 7
0.75
V
V
V
W
Operating temperature
Topr
-30 to +125
°C
Storage temperature
Tstg
-40 to +150
°C
Junction temperature
Tjmax
150 °C
* Specified circuit board : 114.3×76.1×1.6mm3, glass epoxy board.
Caution 1) Absolute maximum ratings represent the value which cannot be exceeded for any length of time.
Caution 2) Even when the device is used within the range of absolute maximum ratings, as a result of continuous usage under high temperature, high current,
high voltage, or drastic temperature change, the reliability of the IC may be degraded. Please contact us for the further details.
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating
Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability.
Recommended Operating Conditions at Ta = 25°C
Parameter
Input voltage range
Symbol
VIN
Conditions
Ratings
4.5 to 32
Unit
V
Semiconductor Components Industries, LLC, 2013
August, 2013
32812 SY 20120207-S00002 No.A2022-1/6

1 page




LA5735MC pdf
LA5735MC
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 currnet to around 500μA.
R1=
1.23V
500μA
2.4kΩ
We recommend values in the range 2.0 to 2.4kΩ
R2=
VOUT
1.23V -1
× R1
The following equation gives the output voltage set by R1 and R2.
VO=
(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. Abnormal oscillation may occur when the C2
capacitance value is small or the equivalent series resistance is small. In this case, addition od the capacitance of C3
enables phase compensation, contributing to stabilization of power supply.
3. Input capacitor: Effective-value current
The AC ripple currents flowing in the input capacitor is large than that in the output capacitor. The equation
expressing the effective-value current is as follows. Use the capacitor within the rated current range.
IC1= Vout (Iout2 (1Vout ) + 1 × ΔIR2 ) [Arms]
Vin Vin 12
4. Output capacitor: Effective-value current
The AC ripple current flowing in the output capacitor is the triabgular 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 300kHz
5. Choke coil L1
Note that choke coil heating due to overload or load shorting may be a problem.The inductance value can 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
×
Ton
=
12
-
5.0 -
0.15
1.0
×
1.58
×
10-6
68μH
T
Ton = ((VIN - VOUT - Vsat)/(VOUT + VF)) + 1
Toff = T - Ton
t : Switching repetition period ··· 3.33μs is assumed for the calculation
VF : Schottky diode forward voltage0.4V is assumed for the calculation
No.A2022-5/6

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