MAX5019/MAX5020
vantage to this is that the MOSFET voltage rating must
be higher and that slope compensation must be provid-
ed to stabilize the inner current loop. The MAX5019
provides internal slope compensation.
Internal Regulators
The internal regulators of the MAX5019/MAX5020
enable initial startup without a lossy startup resistor and
regulate the voltage at the output of a tertiary (bias)
winding to provide power for the IC. At startup V+ is
regulated down to V
CC
to provide bias for the device.
The V
DD
regulator then regulates from the output of the
tertiary winding to V
CC
. This architecture allows the ter-
tiary winding to only have a small filter capacitor at its
output thus eliminating the additional cost of a filter
inductor.
When designing the tertiary winding calculate the num-
ber of turns so the minimum reflected voltage is always
higher than 12.7V. The maximum reflected voltage
must be less than 36V.
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 MAX5019/MAX5020 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:
where C
SS
is the soft-start capacitor as shown in Figure 2.
Operation begins when V
SS_SHDN
ramps above 0.6V.
When soft-start has completed, V
SS_SHDN
is regulated
