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LA5774MP Datasheet(PDF) 4 Page - ON Semiconductor

Part No. LA5774MP
Description  Separately-excited Step-down Switching Regulator
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

LA5774MP Datasheet(HTML) 4 Page - ON Semiconductor

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LA5774MP
No.A0490-4/5
1.5V
0.62V
PWM wave
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:
The VOS pin has the inrush current of 1
μA (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
μA (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
μF
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.
(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 = (
ΔVPWM) × PWMduty + VPWML = 0.88V × 0.23 + 0.62V = 0.82V
(
ΔVPWM 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:
For the set conditions of C = 1
μF and PWMduty = 23%:
VO = 1+
× 1.26V(typ)
R2
R1
PWMduty =
= 23 %
VOUT
VIN − Vsat + VF
Tss =
=
C
× ΔVPWM × PWMduty
i
C
× 0.88 × PWMduty
22
μA
Td =
=
= 28.2 ms
C
× V
i
1
μF × 0.62
22
μA
Tss =
= 9.2ms
1
μF × 0.88V × 0.23
22
μA


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