The LA5744 is a separately-excited step-down switching regulator (variable type).
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Functions
•
High efficiency.
•
Time-base generator (300kHz) incorporated.
•
Current limiter incorporated.
•
Thermal shutdown circuit incorporated.
•
Soft start circuit incorporated.
Specifications
Maximum Ratings
at Ta = 25°C
Parameter
Input voltage
Maximum output current
SW pin application reverse voltage
Allowable power dissipation
Symbol
VIN
IO max
VSW
Pd max1
Pd max2
Operating temperature
Storage temperature
Junction temperature
Topr
Tstg
Tj max
No heat sink
Infinite heat sink
Conditions
Ratings
30
3
-1
1.75
7.5
-30 to +125
-40 to +150
150
Unit
V
A
V
W
W
°C
°C
°C
* Specified circuit board : 76.1×114.3×1.6mm
3
: Copper foil ratio 60% FR4
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
52307 MS PC 20070522-S00004 / 42507 MS PC / 31407 MS PC / 30707 MS PC No.A0717-1/5
LA5744
Electrical Characteristics
at Ta
=
25°C, VO = 5V
Parameter
Reference voltage
Efficiency
Switching frequency
Line regulation
Load regulation
Output voltage temperature
coefficient
Ripple attenuation factor
Current limiter operating voltage
Thermal shutdown operating
temperature
Thermal shutdown Hysteresis
width
* Design target value : No measurement made.
ΔTSD
Designed target value. *
15
°C
RREJ
IS
TSD
f = 100 to 120Hz
VIN = 15V
Designed target value. *
3.1
165
45
dB
A
°C
Symbol
VOS
η
f
ΔV
OLINE
ΔV
OLOAD
ΔV
O/ΔTa
Conditions
min
VIN = 15V, IO = 1.0A
VIN = 15V, IO = 1.0A
VIN = 15V, IO = 1.0A
VIN = 8 to 20V, IO = 1.0A
VIN = 20V, IO = 0.5 to 1.5A
Designed target value. *
240
1.20
Ratings
typ
1.23
83
300
40
10
±0.5
360
100
30
max
1.26
V
%
kHz
mV
mV
mV/°C
Unit
Package Dimensions
unit : mm (typ)
3343
10.0
Pd max -- Ta
10.4MAX
3.6
10.0
4.5
1.3
Allowable power dissipation, Pd max -- W
8.0
7.5
100×100×1.5mm
3
Infinite heat sink
θjc=5°C
2.7
6.0
5.6
50×50×1.5mm
3
Al heat
sink
mounting
torque
39N⋅cm
silicone
grease
applied
40
60
80
100
120
140
160
MSG07021
15.0
(8.8)
15.3
4.0
3.3
2.0
1.75
30×30×1.5mm
3
No heat sink
21.7
0.9
2.7
1
2
5
0.8
(1.6)
1.7
0.4
4.5
8.4
23.0
0
-30 -20
0
20
Ambient temperature, Ta --
°C
SANYO : 220-5HK
Pin Assignment
(1) VIN (2) SWOUT (3) GND (4) VOS (5) SS
No.A0717-2/5
LA5744
Block Diagram
VIN 1
Constant
current
circuit
Reg.
OCP
2 SWOUT
OSC
Reset
Drive
Comp.
TSD
4 VOS
VREF
ON/OFF Comp.
0.4V
5
SS
3
Amp.
GND
Application Circuit Example
L1
22μH
VIN
SWOUT
LA5744
C1
470μF
/50V
D1
SBD
C2
390μF
VOS
SS
C3
1μF
GND
R2
R1
Notes :
1. C3 is for the soft start function. Delete C3 and keep the SS pin open when the soft start function is not necessary.
2.
In some 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.A0717-3/5
LA5744
Protection Circuit Functional Descriptions
1. Overcurrent protection function
The overcurrent protection function detects, on a cycle-by-cycle basis, the output transistor current and turns off that
output transistor current if it exceeds 3.1A.
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 30kHz
if the VOS pin voltage falls below 0.8V.
Note 3 : If the soft start function is not used, the IC will start up with the overcurrent protection function operating. At this
time, the switching frequency will be cut in half. This means that the switching frequency will be 15kHz at
startup.
Note 4 : Since the switching frequency becomes 30kHz when the VOS pin voltage falls under 0.8V, the current capacity is
reduced. If a load is applied with the VOS pin voltage over 0.8V, the inductance value operates at 22μH. If a load
is to be applied when this voltage is under 0.8V, the inductance value must be increased.
Description of Functional Settings
1. Calculation equation to set the output voltage
This IC controls the switching output so that the VOS pin voltage becomes 1.23V (typ).
The equation to set the output voltage is as follows :
R2
VO = 1+
×
1.23V(typ)
R1
The VOS pin has the inrush current of 1μA (typ). Therefore, the error becomes larger when R1 and R2 resistance
values are large.
2. Startup Delay Function
The output voltage rises when the internal voltage reaches 0.4V (typical). Until that point, a capacitor is charged from
an internal 10μA (typical) constant-current supply. The startup delay time can be calculated as shown below
Example : Assume a 1μF capacitor is used.
Td =
C
×
V 1μF
×
0.4
=
= 40 ms
i
10μA
No.A0717-4/5
LA5744
Timing Chart
VIN voltage
30kHz
300kHz
SWOUT voltage
1.23V
0.8V
VOS voltage
0V
Td
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