Diodes ZXLD1320 - Manual (Page 9)

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
supply voltage (VIN), LED forward voltage drop (V
LED
) and circuit configuration, this can provide
up to 24W of output power.
Applications cover both commercial and automotive environments with VIN ranging from 4 to 18V.
• 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
Control is achieved by sensing the LED current in a series resistor (R
M
), connected between the
two inputs of the LED Current Monitor. This generates a proportional current (IMON) that charges
the external integrator capacitor C
FB
. I
MON
is balanced against a reference discharge current
(I
ADJ
) generated at the output of a second voltage to current converter driven from the demand
voltage (V
ADJ
) on the ADJ pin. The difference between I
MON
and I
ADJ
is integrated by C
FB
to
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.