ZXLD1370Q

Document number: DS37117 Rev. 1 - 2

13 of 37

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August 2015

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ZXLD1370Q

Application Information

The ZXLD1370Q 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 21

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. The current ramps up until

an upper threshold value is reached (see Figure 22). At this point

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

to VIN plus the forward voltage, VF, of the 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.

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















V

V

218

.

0

V

REF

ADJ

SENSE

Therefore,



























V

V

R

218

.

0

I

REF

ADJ

S

LED

(Buck mode)

Equation 1

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



























V

V

R

218

.

0

I

REF

ADJ

S

LED

(Buck mode)

Figure 21 Buck Configuration

Figure 22 Operating Waveforms (Buck Mode)