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Offline and DC-DC PWM Controllers for
High-Brightness LED Drivers
Soft-Start
The MAX16801/MAX16802 soft-start feature allows the
LED current to ramp up in a controlled manner. Soft-
I LED = REF
V CC
2V/div
MAX16801
V IN PIN
5V/div
0
100ms/div
Figure 3. V IN and V CC During Startup when Using the
MAX16801 in Bootstrapped Mode
the wake-up level of +23.6V. Once V IN exceeds the
bootstrap UVLO threshold, NDRV begins switching the
MOSFET and transfers energy to the secondary and
tertiary outputs. If the voltage on the tertiary output
builds to a value higher than +9.7V (the bootstrap
UVLO lower threshold), then startup has been accom-
plished and sustained operation commences.
If V IN drops below +9.7V before startup is complete,
the device goes back to low-current UVLO. In this
case, increase C1 in order to store enough energy to
allow for the voltage at the tertiary winding to build up.
100mA/div
start begins after UVLO deasserts. The voltage applied
to the noninverting node of the amplifier ramps from 0
to +1.23V over a 60ms soft-start timeout period. Figure
4 shows a typical 0.5A output current during startup.
Note the staircase increase of the LED current. This is a
result of the digital soft-starting technique used. Unlike
other devices, the reference voltage to the internal
amplifier is soft-started. This method results in superior
control of the LED current.
n-Channel MOSFET Switch Driver
The NDRV pin drives an external n-channel MOSFET.
The NDRV output is supplied by the internal regulator
(V CC ), which is internally set to approximately +9.5V.
For the universal input voltage and applications with a
transformer, the MOSFET used must be able to with-
stand the DC level of the high-line input voltage plus
the reflected voltage at the primary of the transformer.
For most offline applications that use the discontinuous
flyback topology, this requires a MOSFET rated at
600V. NDRV can source/sink in excess of the
650mA/1000mA peak current. Select a MOSFET that
yields acceptable conduction and switching losses.
Internal Error Amplifier
The MAX16801/MAX16802 include an internal error
amplifier that can be used to regulate the LED current
very accurately. For example, see the nonisolated
power supply in Figure 5. Calculate the LED current
using the following equation:
V
R 7
where V REF = +1.23V. The amplifier ’s noninverting
input is internally connected to a digital soft-start circuit
that gradually increases the reference voltage during
startup and is applied to this pin. This forces the LED
current to come up in an orderly and well-defined man-
ner under all conditions.
0
10ms/div
Figure 4. Typical Current Soft-Start During Initial Startup
_______________________________________________________________________________________
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