Offline and DC-DC PWM Controllers for

High-Brightness LED Drivers

10

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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

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

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

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:

For example:

Choose the 15µF standard value.

Assuming C1 > C2, calculate the value of R1 as follows:

at startup (90µA, max).

For example, for the minimum AC input of 85V:

Choose the 120k

Ω 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.

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