DN33

Design Note 33
Issue 1 April 1996
Isolating High Side Switch for Thermal Printer SMPS
R1
L1
+12V
d.c
60µH
68R
2.5W
L2
D1
1N5821
Vin Vsw
Q1
ZTX
949
+24V
d.c
1.5µH
R3
22k
LT1170
Vfb
Vc GND
C1
68µF
CONTROL
SIGNAL
5.0V
Q2
ZTX107
R6
C4
100µF
C5
390µF
C6
100µF
R4
1.2k
470R
R5
10k
GND
C3
100µF
R2
1k
C2
1µF
Figure 1
LT1170 Effected Boost Converter Circuit using a ZTX949 to Isolate Supply.
This circuit has been designed for use in
thermal printers, to isolate the output of
a boost converter from the input supply.
used with the original circuit to provide
an isolated 24V 2A high efficiency power
supply.
This allows the print head to be powered
down when not in use, so that the
thermal paper is not discoloured - this
can be a problem with this converter
topology as even when the converter IC
is disabled, a direct path is available
through the boost inductor L1 and the
Schottky diode D1.
The low VCE(sat) of the ZTX949 (rated at a
BVCEO of 30V, a continuous collector
current of 4.5A and a pulsed current at
20A) enables the device to switch low
duty cycle pulse trains up to 7A without
a heatsink.
The circuit modification includes a low
VCE(sat), high current PNP transistor - the
ZTX949, as a high side switch controlled
via the ZTX107. These components are
Si m il a r is o l a ti ng sw itches can b e
configured using P-Channel MOSFETs,
but due to their lower silicon efficiency
these parts do not permit cost and space
effective designs.
DN33 - 1
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