Datasheet

17Maxim Integrated
Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
MAX5974A/MAX5974B/MAX5974C/MAX5974D
UVLO on Power Source
The enable input has an accurate threshold of 1.26V
(max). For applications that require a UVLO on the
power source, connect a resistive divider from the power
source to EN to GND as shown in Figure 1. A zener
diode between IN and GND is required to prevent the
NDRV and AUXDRV gate-drive voltages from exceeding
20V, the maximum allowed gate voltage of power FETs.
The external zener diode should clamp in the following
range:
Z UVLO(MAX)
20V V V>>
where V
Z
is the zener voltage and V
UVLO(MAX)
is the
maximum wakeup level (16.5V or 8.85V depending on
the device version). An 18V zener diode is the best
choice.
Design the resistive divider by first selecting the value of
R
EN1
to be on the order of 100kω. Then calculate R
EN2
as follows:
_
EN(MAX)
EN2 EN1
S(UVLO) EN(MAX)
V
VR
VV
= ×
where V
EN(MAX)
is the maximum enable threshold volt-
age and is equal to 1.26V and V
S(UVLO)
is the desired
UVLO threshold for the power source, below which the
device is disabled.
The digital output connected to EN should be capable
of withstanding more than the maximum supply voltage.
MCU Control of Enable Input
When using a microcontroller GPIO to control the enable
input, an 18V zener diode is required on IN as shown in
Figure 2.
High-Voltage Logic Control of Enable Input
In the case where EN is externally controlled by a high-
voltage open-drain/collector output (e.g., PGOOD indi-
cator of a powered device controller), connect IN to EN
through a resistor R
EN
and connect EN to an open-drain
or open-collector output as shown in Figure 3. Select
R
EN
such that the voltage at IN, when EN is low, is less
than 20V (i.e., the maximum gate voltage of the main and
AUX FETs):
EN
S(MAX)
EN IN
R
V 20V
RR
×<
+
where V
S(MAX)
is the maximum supply voltage. Obeying
this relationship eliminates the need for an external zener
diode.
The digital output connected to EN should be capable of
withstanding more than 20V.
Figure 1. Programmable UVLO for the Power Source
Figure 2. MCU Control of the Enable Input
R
IN
V
S
R
EN1
DIGITAL
CONTROL
IN
EN
18V
R
EN2
C
IN
N
MAX5974
C
IN
18V
IN
R
IN
V
S
IN
EN
MCU
I/O
MAX5974_