To reduce power dissipation the high-voltage regulator
is disabled when the V
DD
voltage reaches 12.7V. This
greatly reduces power dissipation and improves effi-
ciency. If V
CC
falls below the undervoltage lockout
threshold (V
CC
= 6.6V), the low-voltage regulator is dis-
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+
and V
DD
may be connected to the line voltage provid-
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
MAX5014/MAX5015
Current-Mode PWM Controllers with Integrated
Startup Circuit for Isolated Power Supplies
