The LA5735MC is a separately-excited step-down switching regulator (variable type).
http://onsemi.com
Functions
•
Time-base generator (300kHz) incorporated.
•
Current limiter incorporated.
•
Thermal shutdown circuit incorporated.
Specifications
Absolute Maximum Ratings
at Ta = 25°C
Parameter
Input voltage
SW pin application reverse voltage
VOS pin application voltage
Allowable power dissipation
Operating temperature
Storage temperature
Junction temperature
Symbol
VIN
VSW
VVOS
Pd max
Topr
Tstg
Tjmax
Mounted on a circuit board.*
Conditions
Ratings
34
-1
-0.2 to 7
0.75
-30 to +125
-40 to +150
150
Unit
V
V
V
W
°C
°C
°C
* Specified circuit board : 114.3×76.1×1.6mm
3
, 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
LA5735MC
Electrical Characteristics
at Ta
=
25°C, VIN = 15V
Parameter
Reference voltage
Reference pin bias current
Switching frequency
Short-circuit protection circuit
operating switching frequency
Saturation voltage
Maximum on duty
Minimum on duty
Output leakage current
Supply current
Current limiter operating current
Thermal shutdown operating
temperature
Thermal shutdown Hysteresis
width
* Design target value : Design guarantee values are replaced with electrical measurements, and are not measured by temperature.
ΔTSD
Designed target value. *
15
°C
Vsat
D max
D min
Ilk
Iin
IS
TSD
Designed target value. *
IOUT = 0.3A, VOS = 0V
VOS = 0V
VOS = 5V
SWOUT = -0.4V
VOS = 2V
0.7
165
5
1
100
0
200
10
1.15
V
%
%
μA
mA
A
°C
Symbol
VOS
IFB
fosc
fscp
240
IO = 0.3A
Conditions
min
1.20
Ratings
typ
1.23
1
300
15
max
1.26
2
360
V
μA
kHz
kHz
Unit
Package Dimensions
unit : mm (typ)
3424
4.9
8
1
Pd max -- Ta
Designated board : 114.3×76.1×1.6mm
3
glass epoxy
Allowable power dissipation, Pd max - W
0.8
0.75
Mounted on a board
3.9
6.0
0.375
0.835
0.6
1
1.27
2
0.42
1.75 MAX
0.2
0.4
0.2
0.15
0
--30
0
30
60
90
120
150
0.175
Ambient temperature, Ta - C
SANYO : SOIC8
No.A2022-2/6
LA5735MC
Pin Assignment
NC
NC
GND
NC
VIN
NC SWOUT VOS
Block Diagram
VIN
1
Reg.
OCP
3 SWOUT
OSC
NC
NC
NC
NC
2
Comp.
5
7
8
Reset
Drive
TSD
4 VOS
Amp.
VREF
6
GND
Note : Since the NC pins are not connected within the IC package, they can be used as connection points.
Application Circuit Example
L1
VIN
SWOUT
LA5735MC
+
C1
D1
C3
+
C2
VOS
GND
R1
R2
Note: Insome cases, the output may not turn on if power is applied when a load is connected. If this is a problem, increase
the value of the inductor.
No.A2022-3/6
LA5735MC
Protection Circuit Functional Descriptions
1. Overcurrent protection function
The overcurrent protection function detects, on a pulse-by-pulse basis, the output transistor current and turns off that
output transistor current if it exceeds 0.7A in a pulse-by-pulse manner.
Limit current
Inductor current
SWOUT voltage
2. Short circuit protection function
This IC prevents the current from increasing when the outputs are shorted by setting the switching frequency to 15kHz
if the VOS pin voltage falls below 0.8V.
Note : At startup, since the switching frequency will be 15kHz while the VOS pin voltage is 0.8V or lower, the current
capacity is reduced. If the load is applied at startup and the applications has trouble starting, increase the value of
the inductor to resolve this problem.
Timing Chart
VIN voltage
30kHz
160kHz
SWOUT voltage
1.23V
0.8V
VOS voltage
0V
No.A2022-4/6
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
≈
2.4kΩ
500μA
We recommend values in the range 2.0 to 2.4kΩ
VOUT
R2= 1.23V -1
×
R1
The following equation gives the output voltage set by R1 and R2.
R2
VO= (1+ 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
≤
20kHz
2
× π ×
C2
×
ESR
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=
1
Vout
Vout
)
+ × Δ
IR
2
)
(Iout
2
(1
−
12
Vin
Vin
[Arms]
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.
VOUT (VIN - VOUT)
1
IC2 = 2 3
×
L
×
fsw
×
VIN
√
fsw = Switching frequency
[Arms]
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 - Vsat
×
Ton
ΔIR
12 - 5.0 - 1.0
×
1.58
×
10
-6
0.15
≈
68μH
T
Ton
= ((V
- V
IN OUT - 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
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