RP515x

No.EA-505-251006

12

THEORY OF OPERATION

DC/DC Section

Soft-start Time

Starting-up with CE1 Pin

The IC starts to operate when the CE1 pin voltage (VCE1) exceeds the threshold voltage. The threshold voltage

is preset between CE1

“High” input voltage (VCE1H) and CE1 “Low” input voltage (VCE1L). After the start-up of

the IC, soft-start circuit starts to operate. Then, after a certain period of time, the reference voltage (VREF) in

the IC gradually increases up to the specified value. Switching starts when VREF reaches the preset voltage,

and after that the output voltage rises as VREF increases. Soft-start time (tSTART) indicates the period from the

time soft-start circuit gets activated to the time VREF reaches the specified voltage. tSTART is not always equal

to the turn-on speed of the DC/DC converter. Note that the turn-on speed could be affected by the power

supply capacity, the output current (IOUT), the inductance and the output capacitor value (COUT1).

VCE1H

Soft-start Time

IC Internal Reference Voltage

VCE1L

L

Threshold Level

Lx Voltage

(VCE1)

(VREF)

Soft-start Circuit

operation starts.

(VLX)

Depending on Power Supply,

Load Current, External Components

(VOUT)

Output Voltage

CE1 Pin Input Voltage

Timing Chart: Starting-up with CE1 Pin

Starting-up with Power Supply

After the power-on, when VIN exceeds the UVLO release voltage (VUVLOR), the IC starts to operate. Then, soft-

start circuit starts to operate and after a certain period of time, VREF gradually increases up to the specified

value. Switching starts when VREF reaches the preset voltage, and after that the output voltage rises as VREF

increases. The turn-on speed of VOUT could be affected by following conditions:

1.

The VIN turn-on speed determined by the power supply to the IC and the CIN

2.

The output capacitor value (COUT1) and the output current (IOUT)

Output Voltage

(VOUT)

Input Voltage

(VIN)

VUVLOR

IC Internal Reference Voltage

(VREF)

VUVLOF

Lx Voltage

(VLX)

VSET

VOUT

Depending on Power Supply,

Load Current, External Components

Soft-start Time

Timing Chart: Starting-up with Power Supply