ZXLD1320

Issue 1 - January 2008

9

www.zetex.com

© Zetex Semiconductors plc 2008

Device description

The ZXLD1320 is a buck mode inductive DC-DC converter, with an internal switch, designed for

driving single or multiple LEDs in series up to a total of 1.5A output current. Depending upon

up to 24W of output power.

Applications cover both commercial and automotive environments with VIN ranging from 4 to 18V.

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The device employs a modified Pulse Frequency Modulation (PFM) control scheme, with

variable "ON" and "OFF" time control and adjustable peak switch current limiting.

General device operation (refer to block diagram)

Normal operation

two inputs of the LED Current Monitor. This generates a proportional current (IMON) that charges

produce an error voltage. A comparator takes a summed version of the voltage at the ISENSE pin

and a fraction of this CFB voltage and resets the latch driving the switch when the sum is greater

than 50mV. The switch transistor is turned on by the output of the SR latch, which remains set

until the emitter current in the switch transistor produces a voltage drop Vsense (=50mV nominal)

in external resistor Rsense, defining a preset maximum switch current of 50mV/Rsense.

Operation is such that a rising error voltage on CFB will effectively lower the voltage required on

the ISENSE pin and therefore reset the latch earlier in the switching cycle. This will reduce the

'ON' time of the switch and reduce the peak current in the switch from its preset maximum value.

Similarly, a falling error voltage will reset the latch later and the peak switch current will be

increased. The control loop therefore reduces or increases the energy stored in the coil during

each switching cycle, as necessary, to force the LED current to the set value. This results in high

accuracy, as no error is needed in the LED current to drive the servo to the required region.

The time taken for the coil current to reach the peak value depends on several factors: the supply

voltage, the peak coil current required at that particular LED power and whether the system

operates in "continuous" or "discontinuous" mode.

The time allowed for the coil current to

discharge into the LED is fixed by the 'Variable Off Delay' monostable, whose period is modified

by the power demand signal on the ADJ pin. This monostable determines the time for which the

latch remains reset (switch off) and provides a longer "OFF" period at lower power settings,

helping to keep the parameters within an acceptable range.

Note that the "ON" period and the "OFF" period are set by the supply voltage, LED power and

external components chosen. The frequency is therefore determined by these parameters and is

NOT fixed. The 500mV reference voltage defines the nominal VADJ voltage and this defines the

100% output current. For lower LED currents, the ADJ pin can be-driven from an external dc

voltage (50mV<VADJ<500mV) or a low frequency Pulse Width Modulated (PWM) waveform.