Output voltage adjustment
The ZXSC410/420 are adjustable output converters
allowing the end user the maximum flexibilty. For
adjustable operation a potential divider network is
connected as follows:
The output voltage is determined by the equation:
VV1
RA
RB
OUT FB
=+
where V
FB
=300mV
The resistor values, RA and RB, should be maximised
to improve efficiency and decrease battery drain.
Optimisation can be achieved by providing a minimum
current of I
FB(MAX)
=200nA to the V
FB
pin. Output is
adjustable from V
FB
to the (BR)VCEO of the switching
transistor, Q1.
Note: For the reference designs, RA is assigned the
label R2 and RB the label R3.
Layout issues
Layout is critical for the circuit to function in the most
efficient manner in terms of electrical efficiency,
thermal considerations and noise.
For ‘step-up converters’ there are four main current
loops, the input loop, power-switch loop, rectifier loop
and output loop. The supply charging the input
capacitor forms the input loop. The power-switch loop
is defined when Q1 is ‘on’, current flows from the input
through the inductor, Q1, R
SENSE
and to ground. When
Q1 is ‘off’, the energy stored in the inductor is
transferred to the output capacitor and load via D1,
forming the rectifier loop. The output loop is formed by
the output capacitor supplying the load when Q1 is
switched back off.
To optimise for best performance each of these loops
kept separate from each other and interconnected with
short, thick traces thus minimising parasitic
inductance, capacitance and resistance. Also the
R
SENSE
resistor should be connected, with minimum
trace length, between emitter lead of Q1 and ground,
again minimising stray parasitics.
ZXSC410
ZXSC420
SEMICONDUCTORS
ISSUE 2 - May 2003
7
R
B
V
FB
GND
V
OUT
R
A
STDN
GND
V
CC
DRIVE
SENSE
V
FB
EOR
GND
V
CC
DRIVE
SENSE
V
FB
ZXSC410
SOT23-6
ZXSC420
SOT23-6
CONNECTION DIAGRAMS
