Rainbow Electronics MAX1585 - Manual (Page 25)

Transformerless Inverter for Negative CCD
Bias (AUX2, MAX1585)
On the MAX1585, AUX2 is set up to drive an external P-
channel MOSFET in an inverting configuration. DL2 dri-
ves low to turn on the MOSFET, and FB2 has inverted
polarity and a 0V threshold. This is useful for generating
negative CCD bias without a transformer, particularly
with high pixel-count cameras that have a greater neg-
ative CCD load current. Figures 1 and 8 show such a
configuration for the MAX1585.
Boost with Charge Pump for Positive and
Negative Outputs
Another method of producing bipolar output voltages
without a transformer is with an AUX controller and a
charge-pump circuit as shown in Figure 9. When MOS-
FET Q1 turns off, the voltage at its drain rises to supply
current to V
OUT+ . At the same time, C1 charges to the
voltage V
OUT+
through D1. When the MOSFET turns on,
C1 discharges through D3, thereby charging C3 to V OUT-
minus the drop across D3 to create roughly the same
voltage as V
OUT+ at V OUT- , but with inverted polarity.
If different magnitudes are required for the positive and
negative voltages, a linear regulator can be used at one
of the outputs to achieve the desired voltages. One such
connection is shown in Figure 10. This circuit is some-
what unique in that a positive-output linear regulator is
able to regulate a negative voltage output. It does this by
controlling the charge current flowing to the flying
capacitor rather than directly regulating at the output.
SEPIC Boost-Buck
The MAX1584/MAX1585s’ internal switch step-up and
step-down can be cascaded to make a high-efficiency
boost-buck converter, but it is sometimes desirable to
build a second boost-buck converter with an AUX_
controller.
One type of step-up/step-down converter is the SEPIC,
shown in Figure 11. Inductors L1 and L2 can be sepa-
rate inductors or can be wound on a single core and
coupled like a transformer. Typically, a coupled inductor
improves efficiency since some power is transferred
through the coupling so less power passes through the
coupling capacitor (C2). Likewise, C2 should have low
ESR to improve efficiency. The ripple-current rating must
be greater than the larger of the input and output cur-
rents. The MOSFET (Q1) drain-source voltage rating and
the rectifier (D1) reverse-voltage rating must exceed the
sum of the input and output voltages. Other types of
step-up/step-down circuits are a flyback converter and a
step-up converter followed by a linear regulator.
MAX1584/MAX1585
5-Channel Slim DSC Power Supplies
______________________________________________________________________________________ 25
MAX1585
(PARTIAL)
WHITE
LEDS
FB_
DL_
PVSU
AUX_
PWM
TO
V BATT
NOTE: THIS CIRCUIT CAN
OPERATE WITH AUX1 OR
AUX2 ON THE MAX1584, AND WITH
AUX1 ON THE MAX1585.
62 Ω
(FOR 20mA)
1 µF
1 µF
Figure 6. AUX_ Channel Powering a White LED Step-Up
Current Source
DL_
FB_
PVSU
AUX
PWM
+15V
50mA
CCD+
-7.5V
30mA
CCD-
D2 Q1
TO
V BATT
MAX1584
MAX1585
(PARTIAL)
NOTE: THIS CIRCUIT CAN OPERATE WITH AUX1
OR AUX2 ON THE MAX1584, AND WITH AUX1 ON THE MAX1585.
Figure 7. +15V and -7.5V CCD Bias with Transformer