zxld1601 Zetex Semiconductors plc., zxld1601 Datasheet - Page 8

• Manufacturer Part Number: zxld1601
• Description: Zxld1601 - Adjustable Dc-dc Boost Converter With Internal Switch In Sc70
• Manufacturer: Zetex Semiconductors plc.

V
APPLICATIONS
Setting output voltage
When connected as shown in the typical application
circuit, the ZXLD1601 will produce a nominal default
output of between 26V and 28V. This is set by the
internal potential divider comprising of resistors R1
and R2. (See device block diagram).
The internal potential divider network R1/R2 is
accessible at the FB pin and can be shunted by means
of external resistors to set different nominal output
voltages. The potential divider defines output voltage
according to the relationship:
where VFB = 1.025V.
When using external resistors, these should be chosen
with lower values than the internal resistors to
minimize errors caused by the device to device
variation in absolute value of the internal resistors
(±30% max). The internal resistors have high values in
order to minimize these errors.
The following table gives suggested values for various
output voltages.
Once the nominal output voltage has been set, it can be
adjusted to a lower value by applying a pulse width
modulated (PWM) control signal to the EN pin, using
one of the two methods described below.
PWM adjustment permits the device to be turned on
and the output voltage set by a single logic signal
applied to the EN pin. No external resistors are
required and the amplitude of the control signal is not
critical, providing it conforms to the limits defined in
the electrical characteristics.
ZXLD1601
Required
output voltage
5V
12V
18V
21V
25V
28V
OUT
S E M I C O N D U C T O R S
=
V
FB
⎛
⎜
⎝
1
+
R
R
2
1
⎞
⎟
⎠
External
resistor across
R1
43K
56K
43K
34.8K
27K
40.2K
External resistor
across R2
130K
487K
649K
649K
620K
1.07M
8
1) PWM output voltage adjustment (analogue mode)
During this mode of operation the device operation is
continuous, providing a low ripple output voltage
(V
logic signal applied to the EN pin according to the
relationship:
V
Square wave signals applied to the EN pin, for
example, will turn the device on and produce a nominal
regulated output of 13.5V.
The ZXLD1601 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 120µs after the falling edge of EN. For
continuous PWM mode operation, the frequency of the
control signal must therefore be maintained above
10kHz at all times, to prevent the internal delay circuit
from timing out and switching the device into standby
mode. The maximum frequency applied to EN should
be limited to 100kHz to minimize errors due to internal
switching delays
2) PWM output voltage adjustment (gated mode)
This method of adjustment can be used in applications
where the output ripple is less important than the
supply current. The method of adjustment is the same
as in 1) above, however, during this mode of operation,
the device is gated on and off, providing an average
output voltage (V
cycle (D) of the logic signal applied to the EN pin
according to the relationship:
V
The ripple on this voltage will be determined by the size
of the output capacitor.
The output voltage can be adjusted all the way down to
the input voltage by either method of PWM control, but
for best results, the duty cycle range should be kept
within the specified range. Lower duty cycles will result
in increased output ripple and non-linearity in the
relationship between duty cycle and output voltage. If a
greater control range is required, the nominal output
can be reduced by the use of external resistors before
the PWM signal is applied.
Minimizing output voltage ripple
For applications requiring lower output ripple it may be
necessary to add a small ceramic capacitor in parallel
with R2. A value of 4.7pF is suitable for most output
ranges.
OUT
OUT(AVG)
OUT
) directly proportional to the duty cycle (D) of the
= D x V
= D x V
OUT(nom)
OUT
OUT(nom)
) directly proportional to the duty
ISSUE 3 - AUGUST 2004