ZXLD1321

Issue 1 - January 2008

9

www.zetex.com

© Zetex Semiconductors plc 2008

Device description

The ZXLD1321 is a inductive boost DC-DC converter, with an internal switch, designed for driving

single or multiple LEDs in series up to a total of 1A output current. Depending upon supply

8W of output power.

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

generated at the output of a second voltage to current converter driven from the demand voltage

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. In this modified PFM scheme, the external components can be chosen to keep the

frequency well above the audio range for all extremes of parameters, so no audible whistling

should ever occur.

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.