The LA5724MC is a separately-excited step-down switching regulator (variable type).
http://onsemi.com
Functions
•
Time-base generator (160kHz) 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
Tj max
Mounted on a circuit board.*
Conditions
Ratings
30
-1
-0.2 to 7
0.8
-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 28
Unit
V
Semiconductor Components Industries, LLC, 2013
August, 2013
32812 SY 20120207-S00001 No.A2021-1/6
LA5724MC
Electrical Characteristics
at Ta
=
25°C, VIN = 15V
Parameter
Reference voltage
Reference pin bias current
Switching frequency
Efficiency
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 : No measurement made.
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 = -1V
VOS = 2V
0.7
165
5
1.2
100
0
200
10
V
%
%
μA
mA
A
°C
Symbol
VOS
IFB
fosc
η
fscp
VOUT = 5V, IO = 0.3A
128
IO = 0.3A
Conditions
Ratings
min
1.20
typ
1.23
1
160
82
30
max
1.26
2
192
Unit
V
μA
kHz
%
kHz
Package Dimensions
unit : mm (typ)
3424
4.9
Allowable power dissipation, Pd max - W
8
1
Pd max -- Ta
Designated board : 114.3×76.1×1.6mm
3
glass epoxy
3.9
6.0
0.8
0.75
Mounted on a board
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
0.175
0
30
60
90
120
150
Ambient temperature, Ta - C
SANYO : SOIC8
No.A2021-2/6
LA5724MC
Pin Assignment
NC
NC
GND
NC
LA5724MC
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
LA5724MC
+
C1
D1
C3
+
C2
VOS
GND
R1
R2
Note : Insomecases, 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.A2021-3/6
LA5724MC
Protection Circuit Functional Descriptions
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
Short circuit protection function
This IC prevents the current from increasing when the outputs are shorted by setting the switching frequency to
30kHz if the VOS pin voltage falls below 0.8V.
Note : At startup, since the switching frequency will be 30kHz while the VOS pin voltage is 0.8Vor 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.A2021-4/6
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.
1.23V
R1 =
≈
2.4kΩ
2.0kΩ to 2.4kΩ recommended
500μA
VOUT
R2 = ( 1.23V - 1)
×
R1
The calculation equation gives the output voltage set by R1 and R2.
R2
VOUT = (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. 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 =
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 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.
1
VOUT(VIN-VOUT)
IC2 =
×
[Arms]
L×fSW
×V
IN
2
3
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.
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