Rainbow Electronics MAX686 User Manual
Page 14
MAX686
DAC-Controlled Boost/Inverter
LCD Bias Supply with Internal Switch
14
______________________________________________________________________________________
R6 turns it off when LCDON goes high. R6 and R7 can
be the same value. Choose R7 such that the minimum
base current is greater than 1/50 of the collector current.
For example, assume V
OUT(MIN)
= 12.5V and I
LCD
=
10mA and then determine R7 as follows:
R7
≤
50 x (12.5 - 0.7) / 10mA = 59k
Ω
Remember that the LCD voltage, V
OUTSW
, is the regu-
lated output voltage minus the drop across the PNP
switch (300mV typ).
Connecting V
IN
to V
CC
The MAX686 (V
CC
, V
DD
) and the inductor (V
IN
) can be
powered from the same source as long as the +5.5V
V
CC
maximum limit is not violated. To ensure stability,
connect V
IN
and V
DD
directly to the source, connect
V
CC
to the source through a 100
Ω
resistor (R8), and
bypass V
CC
with a 1µF ceramic capacitor as shown in
Figure 8. Since the supply current is very small, the
voltage drop across R8 is insignificant and does not
degrade performance. The RC isolates V
CC
from the
switching noise created by the inductor and internal
power switch.
Although, in many cases, the MAX686 and the inductor
are powered from the same source, it is often advanta-
geous in battery-powered applications to power the
MAX686 IC (V
CC,
V
DD
) from an available regulated sup-
ply and to power the inductor (V
IN
) directly from a bat-
tery. The MAX686 requires a +2.7V to +5.5V supply at
V
CC
, but the inductor can be powered from voltages as
low as 0.8V, significantly increasing usable battery life.
Layout Considerations
Proper PC board layout is essential due to high current
levels and fast switching waveforms that radiate noise.
It is recommended that initial prototyping be performed
using the MAX686 evaluation kit or equivalent PC
board-based design. Breadboards or proto-boards
should never be used when prototyping switching regu-
lators.
Connect the GND pin, the input bypass-capacitor
ground lead, and the output filter-capacitor ground lead
to a single point (star ground configuration) to minimize
ground noise and improve regulation. Also, minimize
lead lengths to reduce stray capacitance, trace resis-
tance, and radiated noise, with preference given to the
feedback circuit, the ground circuit, and LX. Place R1
and R2 as close to the feedback pin as possible. Place
the bypass capacitors as close to the pins as possible.
Refer to the MAX686 evaluation kit data sheet for an
example of proper board layout.
MAX686
V
DD
V
IN
= 2.7V
TO 5.5V
LX
DACOUT
MBR0530L
R3
C
F
V
OUT
R1
R8
100
Ω
R2
15
µ
F
22
µ
H
1
µ
F
FB
V
CC
Figure 8. Using a Common Supply-Voltage Source