A Product Line of
Diodes Incorporated
ZXLD383
Device Description
The ZXLD383 is a simple PFM, DC-DC controller combined with a high performance internal switching transistor, enabling
the production of a high efficiency boost converter for use in single cell applications. It includes a dual function Enable input
which serves both as an operation inhibit control and an ultra-low voltage drop isolation diode for battery charging purposes
in Garden Light applications. A block diagram is shown for the ZXLD383 in Figure 1.
With power applied and the enable pin held at VCC, an oscillator within the pulse control block forces the internal switching
transistor to switch on to start an energy charge cycle. The low saturation voltage switch pulls the VOUT pin close to ground
which forces the supply voltage across the external inductor L1. This causes a current to build up, storing energy in the
inductor. During this phase, switch current and supply voltage are monitored and used by the pulse control circuit to
determine the optimum drive conditions and on-time. At the end of the energy charge cycle, the internal switch is turned off
rapidly, interrupting the current flow through L1 which causes the voltage on VOUT to rise dramatically. When the voltage on
VOUT reaches the load LED’s forward (on) voltage, the inductor current is transferred from the internal switch to the LED,
starting the energy discharge cycle. With the voltage across the inductor reversed, the current flowing through it (and the
LED) now falls. When the inductor current reaches zero, the voltage on the VOUT pin falls back towards VCC. This action is
sensed by the pulse control circuit and is combined with the output of an off-period timer to initiate the next energy charge
cycle. Except for low level losses, all the energy stored in the inductor during a charge cycle is channeled to the load LED
during the following discharge cycle.
The current fed into the load LED has a sawtooth waveform, the average (DC) value of which is kept constant by the pulse
control circuit for varying supply voltage and temperature. It is possible to change the output current given by the ZXLD383
by changing the value of inductor L1. The larger the inductance of L1, the lower the output current. A table/graph showing
the relationship between inductance and output current is given later in this datasheet. Since the output current of the
ZXLD383 is a sawtooth waveform, its peak value is substantially larger than the DC/average value. The table also provides
this data.
The internal switching transistor has a minimum collector-emitter breakdown voltage of 20V and this sets the maximum
load voltage allowable. The minimum value is set by a feature of the pulse control circuit that requires the load voltage to
be at least 0.5V greater than VCC. (The device will function with load voltages smaller than this but output current regulation
will be impaired.) Higher than nominal load voltages will lower the average (DC) output current generated for a given
inductor value.
The Enable pin inhibits the operation of the output switch if held at a potential of Vcc-0.8V or lower. It also includes a diode
to ground which allows the input to be wired directly to a photocell array that will then both enable operation of the
converter when in darkness and charge the IC’s power source in daylight conditions. The diode function is performed by an
active circuit that gives an ultra low forward voltage drop (typically less than 0.1V at 20mA). This allows the use of a lower
output voltage photocell array (lower cost) without degrading performance.
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www.diodes.com
May 2010
© Diodes Incorporated
ZXLD383
Document number: DS32189 Rev. 3 - 2
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