Separately-Excited Step-Down Switching

Ordering number : ENA0742
LA5774
Monolithic Linear IC
Separately-excited Step-down
Switching Regulator
(Variable Type)
http://onsemi.com
Overview
The LA5774 is a Separately-excited step-down switching regulator (variable type).
Functions
 Low-ESR capacitor with increased reliability applicable as the output smoothing capacitor.
 High efficiency.
 Four external parts.
 Time-base generator (160kHz) incorporated.
 Current limiter incorporated.
 Thermal shutdown circuit incorporated.
 Soft start circuit incorporated.
Specifications
Absolute Maximum Ratings at Ta = 25C
Parameter
Symbol
Maximum Input voltage
VIN max
Maximum Output current
SW pin application reverse voltage
Allowable power dissipation
Conditions
Ratings
Unit
30
V
IO max
3
A
VSW
-1
V
1.75
W
Pd max1
No heat sink
Pd max2
Infinite heat sink
7.5
W
Operating temperature
Topr
-30 to +125
C
Storage temperature
Tstg
-40 to +150
C
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
Semiconductor Components Industries, LLC, 2013
August, 2013
Conditions
Ratings
Unit
4.5 to 28
V
31407 MS PC B8-9090 No.A0742-1/5
LA5774
Electrical Characteristics at Ta  25C, VO = 3.3V
Ratings
Parameter
Symbol
Conditions
Unit
min
Reference voltage
VOS
VIN = 15V, IO = 1.0A

VIN = 15V, IO = 1.0A
f
VIN = 15V, IO = 1.0A
Efficiency
Switching frequency
typ
1.235
max
1.26
1.285
V
78
160
192
kHz
Line regulation
VOLINE
VIN = 8 to 20V, IO = 1A
40
100
mV
Load regulation
VOLOAD
VIN = 15V, IO = 0.5 to 1.5A
10
30
VO/Ta
Output voltage temperature
128
%
mV
0.5
Designed target value. *
mV/C
coefficient
Ripple attenuation factor
RREJ
Current limiter operating voltage
f = 100 to 120Hz
IS
Thermal shutdown operating
45
VIN = 15V
dB
3.1
A
TSD
Designed target value. *
165
C
TSD
Designed target value. *
15
C
temperature
Thermal shutdown Hysteresis
width
* Design target value: No measurement made.
Package Dimensions
unit : mm (typ)
3079C
Pd max -- Ta
10.4MAX
10.0
4.5
2.7
2.7
0.9
5
1
0.8
(1.6)
17.8
21.3
23.0
15.0
1.3
(8.8)
3.6
Allowable power dissipation, Pd max -- W
10.0
1.7
0.45
4.5
8.0
7.5
100×100×1.5mm3
6.0
5.6
50×50×1.5mm3
4.0
3.3
2.0
1.75
Al heat
sink
mounting
torque
39N⋅cm
silicone
grease
applied
30×30×1.5mm3
No heat sink
0
-30 -20
8.4
Infinite heat sink θjc=5°C
0
20
40
60
80
100
Ambient temperature, Ta -- °C
120
140
160
MSG07021
220-5H
Pin Assignment
(1) VIN (2) SWOUT
(3) GND
(4) VOS
(5) SS
No.A0742-2/5
LA5774
Block Diagram
VIN 1
2 SWOUT
Reg.
OCP
Reset
OSC
Drive
Comp.
THD
4 VOS
Amp.
VREF
5
3
SS
GND
Application Circuit Example
L1
68μH
VIN
SWOUT
LA5774
C2
390μF
D1
SBD
C1
470μF
/50V
VOS
SS
GND
R2
R1
C3
1μF
Notes: C3 is for the soft start function. Delete C3 and keep the SS pin open when the soft start function is not necessary.
No.A0742-3/5
LA5774
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 1.26V (typ).
The equation to set the output voltage is as follows:
R2
VO = 1+
× 1.26V(typ)
R1
The VOS pin has the inrush current of 1A (typ). Therefore, the error becomes larger when R1 and R2 resistance
values are large.
2. Start delay function
The SS pin has the internally-connected 22A (typ) constant-current supply. When the voltage of SS pin exceeds the
threshold voltage, the regulator starts operation. As the threshold voltage is 0.62V (typ), the start delay time can be
calculated as follows:
ex. For setting at 1F
Td =
C × V 1μF × 0.62
=
= 28.2 ms
i
22μA
3. Soft start function
The internal PWM waveform has the voltage value as shown in the right.
If down-conversion from the voltage of VIN = 15 V to VIN = 3.3V is to be
made, for example, the PWM-ON duty has the value as shown below.
1.5V
0.62V
PWM wave
VOUT
PWMduty = V − Vsat + VF = 23 %
IN
(Note that calculation is made with Vsat = 1V and VF = 0.2V)
The output voltage of error amplifier, which is 3.3 V, is the value with PWM = 23%, as calculated in the above
equation, so that this voltage is determined as follows:
Ver = (VPWM)  PWMduty + VPWML = 0.88V  0.23 + 0.62V = 0.82V
(VPWM is the PWM amplitude value or 0.88V(typ) while VPWML is the lower limit voltage of PWM
waveform or 0.62V(typ))
SS pin and error amplifier output voltages are designed to prefer the lower voltages, so that VOUT will reach the
designed regulation voltage in timing when the SS pin voltage exceeds the error amplifier output.
Therefore, the soft start time is calculated as follows:
Tss =
C × ΔVPWM × PWMduty C × 0.88 × PWMduty
=
22μA
i
For the set conditions of C = 1F and PWMduty = 23%:
Tss =
1μF × 0.88V × 0.23
= 9.2ms
22μA
No.A0742-4/5
LA5774
Timing Chart
VIN
SWOUT
VOUT
Td
Tss
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PS No.A0742-5/5