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MAX5021 Datasheet(PDF) 6 Page - Maxim Integrated Products

Part No. MAX5021
Description  Current-Mode PWM Controllers for Isolated Power Supplies
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
Logo MAXIM - Maxim Integrated Products

MAX5021 Datasheet(HTML) 6 Page - Maxim Integrated Products

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NDRV did not switch enough times to build up sufficient
voltage across the tertiary output to power the device.
The device will go back into UVLO and will not start. Use
a low-leakage ceramic or film capacitor for C2 and C3.
As a rule of thumb, off-line power supplies keep typical
startup times to less than 500ms even in low-line condi-
tions (85VAC input). Size the startup resistor, RS, to sup-
ply the maximum startup bias of the IC (85µA) plus the
additional current required for charging the capacitors
C2 and C3 in less than 500ms. This resistor dissipates
continuous power in normal operation, despite the fact
that it is only used during the startup sequence.
Therefore it must be chosen to provide enough current
for the low-line condition as well as have an appropriate
power rating for the high-line condition (265VAC). In
most cases, split the value into two resistors connected
in series for the required voltage of approximately
400VDC.
The typical value for C2 and C3 is 220nF. The startup
resistor, RS, provides both the maximum quiescent cur-
rent of 85µA and the charging current for C2 and C3.
Bypass capacitor C3 charges to 9.5V and C2 charges
to 24V all within the desired time period of 500ms, for
an overall average charging current of 15µA. Hence,
the startup resistor must provide a total of at least
100µA. Developing 100µA from an input voltage of
Current-Mode PWM Controllers for Isolated
Power Supplies
6
_______________________________________________________________________________________
D1
AC
L
N
G
CENTRAL SEMICONDUCTOR
CBR1-D100S
AC
L2
470
µH
MAX5022
OPTO
FAIRCHILD
CNY17-3
OPTO
NOTE: ALL RESISTORS ARE 5% UNLESS OTHERWISE SPECIFIED.
CTX03-15256
T1
D2
3A, 40V
ON SEMICONDUCTOR
MBRS340T3
R6
10
C4
150
µF
6.3V
C7
1000pF
C6
0.1
µF
C9
10
µF
400V
C1
10
µF
400V
R3
1k
1%
R4
24.9k
1%
R10
10
8T
3T
480
µH, 60T
R1
360k
R8
1.2k
R2
360k
R11
10
+5V OUT
C5
0.01
µF
C8
8200pF
C2
0.22
µF
N1
INTERNATIONAL RECTIFIER
IRFRC20
C3
0.22
µF
85VAC TO
265VAC IN
D1
250mA, 75V
CENTRAL SEMICONDUCTOR
CMPD914
R5
8.06k
1%
RCS
1.78
1%
R9
240k
TEXAS
INSTRUMENTS
TLV431AIDBV
OPTO
NDRV
U1
CS
VCC
VIN
GND
RS = R1 + R2
Figure 1. Universal 5W Off-Line Standby Power Supply
0
5
15
10
20
25
TIME (ms)
VCC
VIN
0
100
50
150
200
VCC DROPS SLIGHTLY WHEN
NDRV BEGINS SWITCHING
IC COMES OUT OF UVLO
(WAKEUP)
VIN SUPPLIED BY TERTIARY
WINDING (NORMAL OPERATION)
VCC BYPASS CAPACITOR
FULLY CHARGED
VIN SUPPLIED BY C2
Figure 2. VIN and VCC During Startup


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