ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

20 of 42

www.diodes.com

February 2012

© Diodes Incorporated

A Product Line of

Diodes Incorporated

Applications Information

The ZXLD1371 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 DESCRIPTION

a) Buck mode – the most simple buck circuit is shown in Figure 1

Control of the LED current 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 Q1 via the internal Gate

Driver. When the switch is on, the drain voltage of Q1 is

near zero. Current flows from VIN, via Rs, LED, coil and

switch to ground.

This current ramps up until an upper

threshold value is reached (see Figure 2).

At this point

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

schottky diode D1. Current flows via Rs, LED, coil and D1

back to VIN. When the coil 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.

The feedback loop adjusts the

NMOS switch duty cycle to stabilize the LED current in

response to changes in external conditions, including input

voltage and load voltage.

Figure 1. Buck configuration

The average current in the sense resistor, 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 switch duty cycle continuously to force the

average sense resistor current 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 regulates the 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 some conditions.

This depends upon the desired LED current, the coil

inductance and the voltages at the input and the load. Loop

compensation is achieved by a single external capacitor C2,

connected between SHP and SGND.

The control loop sets the duty cycle so that the sense

voltage is

Therefore,

0.218

(Buck mode)

Equation 1

If the ADJ pin is connected to the REF pin, this simplifies to

0.218

(Buck mode).

Figure 2. Operating waveforms (Buck mode)

GATE

voltage

+11V to

15V typ.

0V

Q1

Drain

voltage

VIN + VF

0V

Coil &

LED

current

0A

Sense

voltage

Mean = 218mV