ZXLD1370

ZXLD1370

Document number: DS32165 Rev. 2 - 2

10 of 33

www.diodes.com

May 2010

© Diodes Incorporated

A Product Line of

Diodes Incorporated

Applications Information

The ZXLD1370 is a high accuracy hysteretic inductive buck/boost/buck-boost controller designed to be used with an

external NMOS switch 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 13

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

external NMOS switch transistor M1 via the internal Gate

Rs, LED, coil and switch to ground. This current ramps up

until an upper threshold value is reached.

At this point

GATE goes low, the switch is turned off and the current

current has ramped down to a lower threshold value, GATE

goes high, the switch is turned on again and the cycle of

events repeats, resulting in continuous oscillation.

Figure 13: Buck configuration

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 30%), the frequency may deviate from nominal in extreme conditions. Loop compensation is

achieved by a single external capacitor C2, connected between SHP and SGND.

Figure 14: Operating waveforms (Buck mode)

~15V

0V

V

VIN

V

VIN

-225mV

225mV/Rs

0A

Gate Voltage

ISM Voltage

Coil/LED current

t

OFF

t

ON