UBA3070_2

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 02 — 2 December 2008

5 of 13

NXP Semiconductors

UBA3070

LED backlight driver IC

If the PWM pin is open circuit or not connected, a fault condition is assumed and the

converter will stop switching. Operation will recommence as soon as the fault condition is

removed.

7.3 Masking and valley switching

The mask pin ensures that turn-on of the converter is at a valley during ringing and not at

a peak during freewheeling of the inductor. If the voltage level at the mask pin is typically

above 100 mV, then turn-on of the switching FET is prevented. If the voltage level is below

this 100 mV, then turn-on prevention of the FET is released and at the moment of valley

detection the FET will actually turn on. The signal for the mask pin can be easily derived

from the signal at the switch node as shown in the application diagram. The current

flowing into the mask pin should be lower than the typical stop current of 60

µA.

If pin MASK is open circuit or not connected, a fault condition is assumed and the

converter will stop operating immediately. Operation will recommence as soon as the fault

condition is removed.

7.4 Conversion frequency

The maximum conversion frequency is limited to minimally 145 kHz by an internal

oscillator. Consequently the inductance value should be chosen such that with the given

supply voltage, LED voltage and component spread the conversion frequency is always

smaller than this. There is no limitation for the minimum conversion frequency.

7.5 Minimum on-time

The minimum on-time of the switch mode converter is determined by the leading edge

blanking time of typically 370 ns. The leading edge blanking time is required to prevent

inadvertent switch off after turn on of the FET due to coupling to the sense node.

7.6 Overcurrent protection

Since the converter behaves as a current source it is principally self-protected from

overcurrent. An extra level of protection is added by means of a second trip level at

typically 0.88 V at the sense node to protect against short-circuits in external components

like diodes, inductors, capacitors or LEDs. Upon detection of this abnormal situation the

converter will stop switching.

7.7 Soft start-up (pin SENSE)

To have slow start-up the inductor peak current is slowly increased by the soft start-up

function. This can be achieved by inserting a resistor and a capacitor between the

capacitor to

(about 0.5 V maximum).

The start level and the time constant of the increasing primary current level can be

V

SENSE

I

ch

R

ss

×

=

I

DM

V

sense max

)

(

I

ch

R

ss

×

()

–

R

sense

-------------------------------------------------------------

=

τ

R

ss

C

ss

×

=