Datasheet

18 Maxim Integrated
Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
MAX5974A/MAX5974B/MAX5974C/MAX5974D
Always-On Operation
For always-on operation, connect EN to IN as shown
in Figure 4. No external zener diode is needed for this
configuration.
Bootstrap Undervoltage Lockout
The devices have an internal bootstrap UVLO that is very
useful when designing high-voltage power supplies (see
the Block Diagrams). This allows the device to bootstrap
itself during initial power-up. The MAX5974A/MAX5974C
soft-start when V
IN
exceeds the bootstrap UVLO thresh-
old of V
INUVR
(16V typ).
Because the MAX5974B/MAX5974D are designed for
use with low-voltage power sources such as wall adapt-
ers outputting 12V to 24V, they have a lower UVLO
wake-up threshold of 8.4V.
Startup Operation
The device starts up when the voltage at IN exceeds 16V
(MAX5974A/MAX5974C) or 8.4V (MAX5974B/MAX5974D)
and the enable input voltage is greater than 1.26V.
During normal operation, the voltage at IN is nor-
mally derived from a tertiary winding of the transformer
(MAX5974C/MAX5974D). However, at startup there is
no energy being delivered through the transformer;
hence, a special bootstrap sequence is required. In the
Typical Application Circuits, C
IN
charges through the
startup resistor, R
IN
, to an intermediate voltage. Only
100FA of the current supplied through R
IN
is used by
the ICs, the remaining input current charges C
IN
until
V
IN
reaches the bootstrap UVLO wake-up level. Once
V
IN
exceeds this level, NDRV begins switching the
n-channel MOSFET and transfers energy to the second-
ary and tertiary outputs. If the voltage on the tertiary
output builds to higher than 7V (the bootstrap UVLO
shutdown level), then startup has been accomplished
and sustained operation commences. If V
IN
drops below
7V before startup is complete, the device goes back to
low-current UVLO. In this case, increase the value of C
IN
in order to store enough energy to allow for the voltage
at the tertiary winding to build up.
While the MAX5974A/MAX5974B derive their input volt-
age from the coupled inductor output during normal
operation, the startup behavior is similar to that of the
MAX5974C/MAX5974D.
Soft-Start
A capacitor from SS to GND, C
SS
, programs the soft-
start time. V
SS
controls the oscillator duty cycle during
startup to provide a slow and smooth increase of the
duty cycle to its steady-state value. Calculate the value
of C
SS
as follows:
SS-CH SS
SS
It
C
2V
×
=
where I
SS-CH
(10FA typ) is the current charging C
SS
dur-
ing soft-start and t
SS
is the programmed soft-start time.
A resistor can also be added from the SS pin to GND to
clamp V
SS
< 2V and, hence, program the maximum duty
cycle to be less than 80% (see the Duty-Cycle Clamping
section).
Figure 3. High-Voltage Logic Control of the Enable Input
Figure 4. Always-On Operation
V
S
R
IN
IN
EN
C
IN
MAX5974
DIGITAL
CONTROL
N
R
EN
C
IN
IN
EN
R
IN
V
S
MAX5974_