UBA3070

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© NXP B.V. 2010. All rights reserved.

Product data sheet

Rev. 3 — 9 December 2010

5 of 16

NXP Semiconductors

UBA3070

LED backlight driver IC

7.3 Masking and valley switching

The MASK pin ensures that switch-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 above

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

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

detection the FET is turned on. The signal for the MASK pin can be derived from the

signal at the switch node as shown in Figure 2. The current flowing into the MASK pin

must be lower than the typical stop current of 60

μA.

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

converter stops operating immediately. Operation restarts as soon as the fault condition is

removed.

7.4 Conversion frequency

The maximum conversion frequency of the UBA3070 is limited to 175 kHz. However, due

to production spread, the maximum conversion frequency of an individual UBA3070

sample can be as low as 145 kHz. Therefore, an inductance value must be selected so

that the applications conversion frequency always remains below 145 kHz after taking into

account variations due to the supply voltage, LED string voltage and component spread.

There is no limitation for the minimum conversion frequency.

7.5 Minimum on-time

The LEB time, typically 370 ns is a fixed delay that determines the minimum on-time of the

controller. The LEB time prevents the MOSFET switch from switching off unintentionally

(due to coupling to the sense node) directly after the start of a conversion cycle.

7.6 OverCurrent Protection (OCP)

As the converter acts as a current source it is self-protected from overcurrent by an extra

level of protection, i.e. a second trip level of typically 0.88 V applied to the short-winding/

sense node as shown in Figure 1. This second trip level is designed to protect the

convertor against short-circuits in external components e.g. in the diodes, inductors,

capacitors and/or LEDs. If a short-circuit is detected the convertor stops switching.

7.7 OverTemperature Protection (OTP)

OTP is provided in the UBA3070 IC and functions as follows. When the junction

temperature exceeds the thermal shutdown temperature of typically 140

°C the converter

stops switching.

7.8 Driver

The driver circuit to the gate of the power FET has a current sourcing capability of

approximately 135 mA and a current sinking capability of 560 mA. This capability means

the FET operates more efficiently with fast switch-on and switch-off cycles. A low driver

source current has been chosen to limit the

ΔV/Δt at switch-on thereby reducing EMI and