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-
rent of the UVLO/EN pin. V
ULR2
= +1.28V, I
UVLO
=
50nA (max), V
IN
is the value of the input-supply voltage
where the power supply must start.
where I
UVLO
is the UVLO/EN pin input current, and
V
ULR2
is the UVLO/EN wake-up threshold.
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
MAX16801 attempts to start when V
IN
exceeds 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
reduce current consumption. Once V
IN
reaches
+23.6V, the UVLO circuit turns on both the CPWM and
ILIM comparators, as well as the oscillator, and allows
the output driver to switch. If V
IN
drops below +9.7V,
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
In isolated LED driver applications, V
IN
can be derived
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 V
CC
during startup. Initially, both V
IN
and V
CC
are 0V. After
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 V
CC
voltage reaches approximately +9.5V, while
the voltage across C1 continues rising until it reaches
