UBA3070T NXP [NXP Semiconductors], UBA3070T Datasheet - Page 5

• Manufacturer Part Number: UBA3070T
• Description: LED backlight driver IC
• Manufacturer: NXP [NXP Semiconductors]

NXP Semiconductors
UBA3070_2
Product data sheet
7.3 Masking and valley switching
7.4 Conversion frequency
7.5 Minimum on-time
7.6 Overcurrent protection
7.7 Soft start-up (pin SENSE)
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.
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 A.
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.
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.
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.
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.
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
SENSE-pin (pin 5) and sense resistor R
capacitor to
The start level and the time constant of the increasing primary current level can be
adjusted externally by changing the values of R
I
DM
=
R
=
ss
V
-------------------------------------------------------------
sense max
C
ss
V
SENSE
R
sense
–
=
Rev. 02 — 2 December 2008
I
I
ch
ch
R
R
ss
ss
(about 0.5 V maximum).
sense
. An internal current source charges the
ss
and C
ss
.
LED backlight driver IC
UBA3070
© NXP B.V. 2008. All rights reserved.
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