in applications with higher input DC voltages by imple-

menting resistor-Zener bias (Figure 2a) or transistor-

Zener-resistor bias (Figure 2b).

MAX16801/MAX16802 Undervoltage

Lockout

The MAX16801/MAX16802 have an input voltage

UVLO/EN pin. The threshold of this UVLO is +1.28V.

Before any operation can commence, the voltage on

this pin has to exceed +1.28V. The UVLO circuit keeps

the CPWM comparator, ILIM comparator, oscillator,

and output driver in shutdown to reduce current con-

sumption (see the Functional Diagram). Use this UVLO

function to program the input start voltage. Calculate

the divider resistor values, R2 and R3 (Figure 5), by

using the following formulas:

The value of R3 is calculated to minimize the voltage-

drop error across R2 as a result of the input bias cur-

where the power supply must start.

MAX16801 Bootstrap Undervoltage

Lockout

In addition to the externally programmable UVLO func-

tion offered in both the MAX16801/MAX16802, the

MAX16801 has an additional internal bootstrap UVLO

that is very useful when designing high-voltage LED

drivers (see the Functional Diagram). This allows the

device to bootstrap itself during initial power-up. The

bootstrap UVLO threshold of +23.6V. During startup,

the UVLO circuit keeps the CPWM comparator, ILIM

comparator, oscillator, and output driver shut down to

+23.6V, the UVLO circuit turns on both the CPWM and

ILIM comparators, as well as the oscillator, and allows

the UVLO circuit will shut down the CPWM comparator,

ILIM comparator, oscillator, and output driver thereby

returning the MAX16801 to the startup mode.

MAX16801 Startup Operation

from a tertiary winding of a transformer. However, at

startup there is no energy delivered through the trans-

former. Therefore, a special bootstrap sequence is

required. Figure 3 shows the voltages on IN and VCC

the line voltage is applied, C1 charges through the

startup resistor R1 to an intermediate voltage. At this

point, the internal regulator begins charging C2 (see

Figure 5). The MAX16801 uses only 45µA of the current

supplied by R1, and the remaining input current

charges C1 and C2. The charging of C2 stops when

the VCC voltage reaches approximately +9.5V, while

the voltage across C1 continues rising until it reaches

R

VV

V

R

IN

ULR

ULR

23

2

2

=×

−

R

VV

IV

V

ULR

IN

UVLO

IN

ULR

3

500

2

2

≅

×

×

()

−

Off-Line and DC-DC PWM Controllers for

High-Brightness LED Drivers

8

_______________________________________________________________________________________

IN

VDC

R

D

(a)

MAX16802A

IN

VDC

Q

R

DC

(b)

MAX16802A

Figure 2. (a) Resistor-Zener and (b) Transistor-Zener-Resistor Bias Arrangements