ZXLD1374 datasheet(15/37 Pages) DIODES

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ZXLD1374

60V HIGH ACCURACY 1.5A BUCK/BOOST/BUCK-BOOST

LED DRIVER CONVERTER WITH AEC-Q100

ZXLD1374

Document number: DS35032 Rev. 2 - 2

15 of 37

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March 2011

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Diodes Incorporated

Applications Information (Continued)

b) Boost and Buck-boost modes

A basic ZXLD1374 application circuit for Buck-boost and Boost modes is shown in Figure 28.

Control in Boost and Buck-boost mode is achieved by

sensing the coil current in the series resistor Rs,

connected between the two inputs of a current monitor

within the control loop block.

An output from the control loop drives the input of a

comparator which drives the gate of the internal NMOS

switch transistor. In Boost and Buck-boost modes, when

the switch is on, current flows from VIN, via Rs, coil and

switch to ground. This current ramps up until an upper

threshold value is reached.

At this point the switch is

turned off and the current flows via Rs, coil, D1 and LED

back to VIN (Buck-boost mode), or GND (Boost mode).

When the coil current has ramped down to a lower

threshold value the switch is turned on again and the

cycle of events repeats, resulting in continuous oscillation.

The average current in the coil is equal to the average of

the maximum and minimum threshold currents and the

ripple current (hysteresis) is equal to the difference

between the thresholds.

Figure 28. Boost and Buck-boost Configuration

The average current in the LED is always less than the

average current in the coil and the ratio between these

currents is set by the values of external resistors RGI1 and

RGI2.

The peak LED current is equal to the peak coil

current.

The control loop maintains the average LED

current at the set level by adjusting the thresholds and the

hysteresis continuously to force the average current in the

coil to the value demanded by the voltage on the ADJ and

GI pins. This minimizes variation in output current with

changes in operating conditions. Loop compensation is

achieved by a single external capacitor C2, connected

between SHP and SGND.

For more detailed descriptions of device operation and for

choosing external components, please refer to the

application circuits and descriptions in the later sections of

this specification.

Figure 29. Operating Waveforms

(Boost and Buck-boost Modes)

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