To reduce power dissipation the high-voltage regulator

greatly reduces power dissipation and improves effi-

abled, and soft-start is reinitiated. In undervoltage lock-

out the MOSFET driver output (NDRV) is held low.

If the input voltage range is between 13V and 36V, V+

ed that the maximum power dissipation is not exceed-

ed. This eliminates the need for a tertiary winding.

Undervoltage Lockout (UVLO), Soft-Start,

and Shutdown

The soft-start feature of the MAX5014/MAX5015 allows

the load voltage to ramp up in a controlled manner,

thus eliminating output voltage overshoot.

While the part is in UVLO, the capacitor connected to

the SS_SHDN pin is discharged. Upon coming out of

UVLO an internal current source starts charging the

capacitor to initiate the soft-start cycle. Use the follow-

ing equation to calculate total soft-start time:

_______________________________________________________________________________________

9

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

MAX5014

V+

SS_SHDN

NDRV

CS

GND

OPTO

OPTOCOUPLER

100

Ω

M1

4.7nF

250VAC

TLV431

220

Ω

Figure 3. Flyback Converter