ZXLD1374

ZXLD1374

Document number: DS35032 Rev. 1 - 2

Page 13 of 35

www.diodes.com

October 2010

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Applications Information

The ZXLD1374 is a high accuracy hysteretic inductive Buck/Boost/Buck-boost converter with an internal NMOS switch

designed to be used for current-driving single or multiple series-connected LEDs. The device can be configured to operate

in Buck, Boost, or Buck-boost modes by suitable configuration of the external components as shown in the schematics

shown in the device operation description.

Device Operation

a) Buck mode

The most simple Buck circuit is shown in Figure 26

LED current control in Buck mode is achieved by sensing the

coil current in the sense 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.

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, LED, coil and D1 back

threshold value the switch is turned on again and the cycle of

events repeats, resulting in continuous oscillation.

The average current in the LED and coil is equal to the

average of the maximum and minimum threshold currents.

The ripple current (hysteresis) is equal to the difference

between the thresholds.

The control loop maintains the average LED current at the set

level by adjusting the thresholds continuously to force the

average current in the coil to the value demanded by the

voltage on the ADJ pin. This minimizes variation in output

current with changes in operating conditions.

The control loop also attempts to minimize changes in

switching frequency by varying the level of hysteresis. The

hysteresis has a defined minimum (typ 5%) and a maximum

(typ 20%), the frequency may deviate from nominal in

extreme conditions. Loop compensation is achieved by a

single external capacitor C1, connected between SHP and

SGND.

VAUX VIN

ISM

PWM

GI

LX

ADJ

LX

REF

FLAG

TADJ

NC x4

SHP STATUS SGND

PGND

L1

D1

C2

C1

VIN

GND

Rs

LED1

LEDn

Figure 26. Buck Configuration

Figure 27. Operating Waveforms (Buck Mode)