ZXLD381
Device Description
The ZXLD381 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. A block diagram is shown for the ZXLD381 in Fig 1.
VCC
L1
ZXLD 381
VOUT
Pulse
Cont rol
LED
1. 5V
Con
Coff
Rsense
GND
Figure 1 ZXLD381 Block Diagram
When power is applied, 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
V
pin close to ground which forces the supply voltage across the external inductor L1. This
OUT
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 V
to rise
OUT
dramatically. When the voltage on V
reaches the load LED's forward (on) voltage, the inductor
OUT
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 V
pin falls back towards V
.
OUT
CC
This action is sensed by the pulse control circuit, which initiates the next energy charge cycle.
Except for low level losses, all the energy stored in the inductor during a charge cycle will be
channelled 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 ZXLD381 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
Issue 1 - April 2008
© Zetex Semiconductors plc 2008
4
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