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Offline and DC-DC PWM Controllers for
High-Brightness LED Drivers
I C 1 = SUVR
V IN ( MIN ) ? V SUVR
I C 1 + I START
Applications Information
Startup Time Considerations for High-
Brightness LED Drivers Using MAX16801
The IN bypass capacitor C1 supplies current immedi-
ately after wake-up (Figure 5). The size of C1 and the
connection configuration of the tertiary winding deter-
mine the number of cycles available for startup. Large
values of C1 increase the startup time but also supply
gate charge for more cycles during initial startup. If the
value of C1 is too small, V IN drops below +9.7V
because NDRV does not have enough time to switch
and build up sufficient voltage across the tertiary wind-
ing that powers the device. The device goes back into
Assuming C1 > C2, calculate the value of R1 as follows:
V × C 1
( 500 ms )
R 1 =
where V IN(MIN) is the minimum input supply voltage for
the application, V SUVR is the bootstrap UVLO wake-up
level (+23.6V, max), and I START is the IN supply current
at startup (90μA, max).
For example, for the minimum AC input of 85V:
( 24 V ) × ( 15 μF )
( 500 ms )
= 119 k ?
UVLO and does not start. Use low-leakage capacitors
for C1 and C2.
Assuming that offline LED drivers keep typical startup
times to less than 500ms even in low-line conditions
(85VAC input for universal offline applications), size the
startup resistor R1 to supply both the maximum startup
I C1 =
R 1 =
= 0 . 72 mA
120 V ? 2 4 V
( 0 . 72 mA + ( 90 μ A ) )
C 1 = ( 1 . 4 mA + 2 . 1 mA ) × ( 60 ms ) = 17 . 5 μ F
bias of the device (90μA, worst case) and the charging
current for C1 and C2. The bypass capacitor C2 must
charge to +9.5V and C1 to +24V, all within the desired
time period of 500ms.
Because of the internal 60ms soft-start time of the
MAX16801, C1 must store enough charge to deliver
current to the device for at least this much time. To cal-
culate the approximate amount of capacitance
required, use the following formula:
I g = Q gtot × f SW
C 1 = ( I IN + I g ) ( t SS )
V HYST
where I IN is the MAX16801’s internal supply current
after startup (1.4mA), Q gtot is the total gate charge for
Q1, f SW is the MAX16801 ’s switching frequency
(262kHz), V HYST is the bootstrap UVLO hysteresis
(11.9V) and t SS is the internal soft-start time (60ms).
For example:
Ig = ( 8 nC ) × ( 262 kHz ) = 2 . 1 mA
( 12 V )
Choose the 15μF standard value.
Choose the 120k ? standard value.
Choose a higher value for R1 than the one calculated
above if longer startup time can be tolerated in order to
minimize power loss on this resistor.
The above startup method is applicable to a circuit sim-
ilar to the one shown in Figure 5. 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 voltage of C1 from
+22V to +10V must be greater than the soft-start time of
60ms.
Another method of bootstrapping the circuit is to have a
separate bias winding than the one used for regulating
the output voltage and to connect the bias winding so
that it is in phase with the MOSFET ON time (see Figure
9). In this case, the amount of capacitance required is
much smaller.
However, in this mode, the input voltage range has to
be less than 2:1. Another consideration is whether the
bias winding is in phase with the output. If so, the LED
driver circuit hiccups and soft-starts under output short-
circuit conditions. However, this property is lost if the
bias winding is in phase with the MOSFET ON time.
10
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