Diodes ZXLD1322 - Manual (Page 8)

ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
8 of 17
www.diodes.com
April 2010
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A Product Line of
Diodes Incorporated
Device Description
The ZXLD1322 is a buck/boost mode inductive DC-DC converter, with an internal switch, designed for driving single or
multiple LEDs in series up to a total of 700mA output current. Depending upon supply voltage (V
IN
), LED forward voltage
drop (V
LED
) and circuit configuration, this can provide up to 12W of output power.
Applications cover V
IN
ranging from 2.5V to 15V.
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 (I
MON
) that charges the external integrator capacitor CFB. 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 CFB to
produce an error voltage. A comparator takes a summed version of the voltage at the I
SENSE
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 V
SENSE
(=50mV nominal) in external resistor R
SENSE
, defining a preset maximum switch current of 50mV/R
SENCE
. Operation is such
that a rising error voltage on CFB will effectively lower the voltage required on the I
SENSE
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 V
ADJ
voltage and this defines the 100% output current. For lower LED
currents, the ADJ pin can be-driven from an external DC voltage (50mV<V
ADJ
<500mV) or a low frequency Pulse Width
Modulated (PWM) waveform.
Low voltage operation (start-up mode)
For supply voltages below 2V, the normal control loop will have insufficient headroom to operate reliably. This condition is
detected by the 'under-voltage comparator', which compares a fraction of the internal supply voltage (V
CC
) against V
REF
.
When the comparator output is active (V
CC
<1.8V), the output of the normal switch drive circuit is disabled and an alternative
'Start-up oscillator and driver' enabled. The start-up oscillator provides a nominal 50kHz fixed frequency drive signal to the
base of the switch transistor, which is independent of V
ADJ
and the voltage on CFB. Under low voltage conditions, the peak
current in the coil ramps to approximately 25% of the normal value and the "OFF" time is fixed.
The low voltage start-up mode allows the device to operate down to 1.2V nominal. This allows the chip to work from a single
cell.