LA5757TP

No.A1439-4/6

1.5V

0.62V

PWM wave

2. Start delay function

The SS pin has the internally-connected 10

μA (typ) constant-current supply. When the voltage of SS pin exceeds

the threshold voltage, the regulator starts operation. As the threshold is 0.62V(typ), the start delay time can be

calculated as follows:

ex. For setting at 1

μF

Td =

=

= 28.2 msec

C

× V

i

1

μ × 0.62

22

μ

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 = 15V to 3.3V output to be made,

for example, the PWM-ON duty has the value as shown below.

PWMduty =

= 25%

VOUT + VF

VIN - Vsat + VF

(Note that calculation is made with Vsat = 1V and VF = 0.2V)

The output voltage of error amplifier, which is 3.3V, is the value with PWM = 25%, as calculated in the above

equation, so that this voltage is determined as follows:

Ver = (

ΔVPWM) × PWMduty + VPWML = 0.88V × 0.25 + 0.62V = 0.84V

(

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

strt time is calculated as follows:

Tss =

=

C

× ΔVPWM × PWMduty

i

C

× 0.88 × PWMduty

22

μA

For the set conditions of C = 1

μF and PWMduty = 25%:

Tss =

= 10msec

1

μ × 0.88V × 0.25

22

μA