NXP Semiconductors

11

34845

Figure 5. Power up sequence case 2, VIN applied after the ENABLE and PWM signals.

VIN ramp up time between 0 V and 5.0 V should be not higher than 2.0 ms

5.2.2

Boost converter

The boost converter uses a Dynamic Headroom Control (DHC) loop to automatically set the output voltage needed to drive the LED

strings. The DHC is designed to operate under specific pulse width conditions in the LED drivers. It operates for pulse widths higher than

400 ns. If the pulse widths are shorter than specified, the DHC circuit does not operate and the voltage across the LED drivers increase

to a value given by the OVP, minus the total LED voltage in the LED string. It is therefore imperative to select the proper OVP level to

avoid exceeding the max off state voltage of the LED drivers (45 V).

The boost operates in current mode and is compensated externally through a type 2 network on the COMP pin. A modification of the

compensation network is suggested to minimize the amplitude of the ripple at VOUT. The details of the suggested compensation network

are shown in Figure 10 and Figure 11.

An integrated 2.0 A minimum FET supplies the required output current. An overcurrent protection circuit limits the output current cycle-

by-cycle to IOCP. If the condition exists longer than 10 ms, then the device shuts down. The frequency of the boost converter is internally

set to 300 kHz or 600 kHz, depending on the device’s version.

The boost also includes a soft start circuit. Each time the IC comes out of shutdown mode, the soft start period lasts for tSS.

Overvoltage protection is also included. The device has an internally fixed OVP value of 60 V (typical) which serves as a secondary fault

protection mechanism, in the event the externally programmed OVP fails (i.e. resistor divider opens up). While the internal 60 V OVP

detector can be used exclusively without the external OVP network, this is only recommended for applications where the LED string

voltage approaches 55 V or more. The OVP level can be set by using an external resistor divider connected between the output voltage

and ground with its output connected to the OVP pin. The OVP can be set up to 60 V by varying the resistor divider to match the OVP

internal reference of 6.9 V (typical).

5.2.3

LED driver

The six channel LED driver provides current matching for six LED strings to within

±2% maximum. The current in the strings is set using

a resistor tied to GND from the ISET pin. The LED current level is given by the equation: RSET = 153/ ILED. The accuracy of the RSET

resistor should be 0.1% for best performance.

5.2.4

LED error detect

If an LED is open, the output voltage ramps to the OVP level. If there is still no current in the LED string, the LED channel is turned off

and the output voltage ramps back down to normal operating level.

If LEDs are shorted and the voltage in any of the channels is greater than the SFDV threshold (7.0 V typical), then the device turns off this

channel. However if the on-time of the channels is less than 10

μs, the SFDV circuit does not disable any of the channels, regardless of

the voltage across them. All the LED errors can be cleared by recycling the EN pin or applying a complete power-on-reset (POR).

VIN

EN

PWM

VOUT

Boost

Soft Start

5V

2ms

VIN ramp

UVLO Rising