Rainbow Electronics MAX5068 - Manual (Page 10)

MAX5068
Use the MAX5068C/D/E/F in forward converter applica-
tions with greater than 50% duty cycle. The large duty
cycle results in much lower operating primary RMS cur-
rent through the MOSFET switch and, in most cases,
requires a smaller output filter capacitor. The major dis-
advantage to this is that the MOSFET voltage rating
must be higher. The MAX5068C/D/E/F capacitor
adjustable-slope-compensation feature allows for easy
stabilization of the inner current loop.
Undervoltage Lockout
The MAX5068 features an input voltage UVLO/EN func-
tion to enable the PWM controller before any operation
can begin. The MAX5068C/E shut down if the voltage
at UVLO/EN falls below its 1.18V threshold. The
MAX5068A/B/D/F also incorporate an UVLO hysteresis
input to set the desired turn-off voltage.
MAX5068C/E UVLO Adjustment
The MAX5068C/E have an input voltage UVLO/EN with
a 1.231V threshold. Before any operation can com-
mence, the UVLO/EN voltage must exceed the 1.231V
threshold. The UVLO circuit keeps the PWM compara-
tor, ILIM comparator, oscillator, and output driver shut
down to reduce current consumption (see the
Functional Diagram).
Calculate R6 in Figure 2 by using the following formula:
where V ULR2 is the UVLO/EN’s 1.231V rising threshold
and V ON is the desired startup voltage. Choose an R7
value in the 20k
Ω
range.
After a successful startup, the MAX5068C/E shut down if
the voltage at UVLO/EN drops below its 1.18V threshold.
R
V
V
R
ON
ULR
6 1 7
2
=

 

× −
High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
10 ______________________________________________________________________________________
MAX5068A
IN
NDRV
CS
PGND
FB V CC
AGND
REG5
UVLO/EN
RT
HYST
DT
FLTINT
D2
R1
C1
Q1
COMP
V
IN
C2
C3
C4
R3
R4
R6
R HYST
R7
SYNC
R9
R8
R2
R5 C5
R CS
VOUT
D1
C6
Figure 1. Nonisolated Power Supply with Programmable Input Supply Voltage