Rainbow Electronics MAX5068 - Manual (Page 13)

Assuming C1 > C3, calculate the value of R1 as follows:
where V SUVR is the bootstrap UVLO wakeup level
(23.6V max), V
IN(MIN)
is the minimum input supply volt-
age for the application (36V for telecom), and I START is
the V IN supply current at startup (90µA, max).
For example:
To minimize power loss on this resistor, choose a high-
er value for R1 than the one calculated above (if a
longer startup time can be tolerated).
The above startup method is applicable to a circuit sim-
ilar to the one shown in Figure 1. In this circuit, the ter-
tiary winding has the same phase as the output
windings. Thus, the voltage on the tertiary winding at
any given time is proportional to the output voltage and
goes through the same soft-start period as the output
voltage. The minimum discharge time of C1 from 22V to
10V must be greater than the soft-start time (t
SS ).
Oscillator/Switching Frequency
Use an external resistor at RT to program the MAX5068
internal oscillator frequency from 50kHz to 2.5MHz. The
MAX5068A/B output switching frequency is one-half of
the programmed oscillator frequency with a 50% duty
cycle. The MAX5068C/D/E/F output switching frequen-
cy is one-quarter of the programmed oscillator frequen-
cy with a 75% duty cycle.
I
V x F
ms
A
R
V V
A A
k
C1
24 2 2
500
106
1
36 12
106 90
122 4
.
.
= =
≅
+
=
−
µ
µ
µ µ
Ω
I
V C
ms
R
V x V
I I
C
SUVR
IN MIN SUVR
C START
1
1
1
500
1
0 5 .
( )
=
×
≅
−
+
MAX5068
High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
______________________________________________________________________________________ 13
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
R2
R5 C5
R CS
VOUT
D1
R11
C6
C10
C7
R12
R13
MAX8515
R9
R10 R8
V
CC
PS2913
Figure 6. Secondary-Side, Regulated, Isolated Power Supply