Rainbow Electronics MAX5974D - Manual (Page 16)

MAX5974A/MAX5974B/MAX5974C/MAX5974D
Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
16
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 (20V 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 10V.
Startup Operation
The device starts up when the voltage at IN exceeds
20V (MAX5974A/MAX5974C) or 10V (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
Figure 1. Programmable UVLO for the Power Source
Figure 2. External Control of the Enable Input
R IN
V S
R EN1
DIGITAL
CONTROL
IN
EN
R EN2
C IN
N
MAX5974
V S
R IN
IN
EN
C IN
MAX5974
DIGITAL
CONTROL
N