zxld1601 Zetex Semiconductors plc., zxld1601 Datasheet - Page 5

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

Device Description
The device is a PFM flyback dc-dc boost converter,
working in discontinuous mode.
With reference to the chip block diagram and typical
application circuit, the operation of the device is as
follows:
Control loop
When ‘EN’ is high, the control circuits become active
and the low side of the coil (L1) is switched to ground
via NDMOS transistor (MN). The current in L1 is
allowed to build up to an internally defined level
(nominally 320mA) before MN is turned off. The energy
stored in L1 is then transferred to the output capacitor
(C2) via Schottky diode (D1). The output voltage is
sensed at pin ‘V
(which may be shunted externally at pin ‘FB’) and
compared to a reference voltage V
nominal). A comparator senses when the output
voltage is above that set by the reference and its output
is used to control the ‘off’ time of the output switch. The
control loop is self-oscillating, producing pulses of up
to 5µs maximum duration (switch ‘on’), at a frequency
that varies in proportion to the output current. The
feedback loop maintains a voltage of VREF at the FB pin
and therefore defines a maximum output voltage equal
to V
output switch is fixed at 0.5µs nominal, to allow time
for the coil’s energy to be dissipated before the switch
is turned on again. This maintains stable and efficient
operation in discontinuous mode.
ISSUE 3 - AUGUST 2004
REF
*(R1+R2)/R1. The minimum ‘off’ time of the
SENSE
’ by internal resistors R1 and R2
REF
(1.025V
5
Filtered PWM operation
The input of an internal low pass filter is switched to
V
when the EN pin is low. The output of this filter drives
the comparator within the control loop. A continuous
high state on EN therefore provides a filtered voltage of
value Vref to the comparator. However, by varying the
duty cycle of the EN signal at a suitably high frequency
(f>10kHz), the control loop will see a voltage, that has
an average value equal to the duty cycle multiplied by
V
voltage to a lower value. It also allows the device to be
both turned on and adjusted with a single signal at the
‘EN’ pin. The output during this mode of operation will
be a dc voltage equal to V
Gated PWM operation
The internal circuitry of the ZXLD1601 is turned off
when no signal is present on the ‘EN’ pin for more than
120µs (nominal). A low frequency signal applied to the
EN pin will therefore gate the device ‘on’ and ‘off’ at the
gating frequency and the duty cycle of this signal can
be varied to provide an average output equal to V
*(R1+R2)/R1 x duty cycle. For best accuracy, the gating
frequency should be made as low as possible (e.g.
below 1kHz), such that the turn off delay of the chip is
only a small proportion of the gating period
Further details of setting output current are given in the
application notes.
REF
REF
. This provides a means of adjusting the output
when the EN pin is high and switched to ground
REF
*(R1+R2)/R1 x duty cycle.
ZXLD1601
S E M I C O N D U C T O R S
REF