Gated mode
The ZXLB1600 contains a timing circuit that switches
the device on a few microseconds after the application
of a rising edge to EN and turns it back off again
nominally 120s after the falling edge of EN. So, if a
lowerfrequencyof1kHzorlessisappliedtotheENpin,
the device will be gated on and off at a duty cycle (D)
corresponding to that of the input signal. The average
output voltage is then given by:
V
OUT(avg)
~ 28 x D
Output voltage can be adjusted all the way down to the
input voltage by means of PWM control, but for best
results, the duty cycle range should be kept within the
specified range of 0.4 to 1. Lower duty cycles may
result in increased output ripple and non-linearity in
the relationship between duty cycle and output
voltage. If a greater control range, or reduced ripple is
required, the nominal output can be adjusted by one of
the other methods before the PWM signal is applied.
Negative output
The ZXLB1600 can be used to provide a negative
output voltage (in addition to the normal positive
output) as shown in the application circuit below. In
this circuit, the external resistors R3 an R4 are used to
set the output voltage to 22V as described in the
previous section. These resistors and output capacitor
C2 have relatively low values in this circuit in order to
give a short time constant. This improves the
regulation of the negative voltage.
Capacitor selection
A low ESR ceramic capacitor grounded close to the
GND pin of the package is recommended at the output
of the device. Surface mount types offer the best
performance due to their lower inductance. A
minimum value of 1F is advised, although higher
values will lower switching frequency and improve
efficiency especially at lower load currents. A higher
value will also minimize ripple when using the device
to provide an adjustable dc output voltage.
A goodquality, low ESR capacitor should also be used for
input decoupling, as theESR ofthis capacitoris effectively
in series with the source impedance and lowers overall
efficiency. This capacitor has to supply the relatively high
peak current to the coil and smooth the current ripple on
the input supply. A minimum value of 3.3F is acceptable
if the input source isclose to the device, but higher values
are recommended at lower input voltages, when the
source impedance is high. The input capacitor should be
mounted as close as possible to the IC.
For maximum stability over temperature, capacitors
with X7R dielectric are recommended, as these have a
muchsmallertemperaturecoefficientthanothertypes.
ZXLB1600
ISSUE 3 - SEPTEMBER 2003
12
SEMICONDUCTORS
LAST TIME BUY
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