MAX16801A Datasheet, PDF(9/15 Page) Maxim Integrated Products – Off-Line and DC-DC PWM Controllers for High-Brightness LED Drivers

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

MAX16801A Datasheet, PDF (9/15 Pages) Maxim Integrated Products – Off-Line and DC-DC PWM Controllers for High-Brightness LED Drivers

◁

Off-Line and DC-DC PWM Controllers for

High-Brightness LED Drivers

VCC

2V/div

MAX16801

VIN PIN

5V/div

100ms/div

Figure 3. VIN and VCC During Startup when Using the

MAX16801 in Bootstrapped Mode

the wake-up level of +23.6V. Once VIN 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 VIN 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

10ms/div

Figure 4. Typical Current Soft-Start During Initial Startup

Soft-Start

The MAX16801/MAX16802 soft-start feature allows the

LED current to ramp up in a controlled manner. Soft-

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

(VCC), 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 off-line 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:

ILED

VREF

where VREF = +1.23V. The amplifierâs 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.

_______________________________________________________________________________________ 9

▷