MAX1566-MAX1567 Datasheet, PDF (32/35 Pages) Maxim Integrated Products – Six-Channel, High-Efficiency, Digital Camera Power Supplies

Six-Channel, High-Efficiency, Digital
Camera Power Supplies
In that case:
Place:
C_C = (V_IN / V_RAMP) [V_REF /
(V_REF + |V_OUT|)] [g_M / (2π x f_C)]
1 / (2π x R_C x C_C) = 1 / (2π x R_LOAD x C_OUT), so that
R_C = R_LOAD x C_OUT / C_C
Or, reduce the inductor value for discontinuous operation.
Applications Information
Typical Operating Circuits
Figures 1, 2, and 3 show connections for AA and Li+
battery arrangements. Figures 7–13 show various con-
nections for the AUX1, 2, and 3 controllers. Figures 15,
16, and 17 show various connections for the SDOK,
AUX1OK, and SCF outputs.
Figure 1. Typical Operating Circuit for One Li+ Cell
In this connection, the main converter is operated as a
step-down (SUSD = GND) and is powered from PVSU.
This provides boost-buck operation for the main 3.3V
output so a regulated output is maintained over the Li+
2.7V to 4.2V cell voltage range. The compound efficien-
cy from the battery to the 3.3V output reaches 90%.
The step-down 1.8V (core) output is powered directly
from V_BATT.
The CCD and LCD voltages are generated with a trans-
formerless design. AUX1 generates +15V for CCD posi-
tive and LCD bias. The MAX1567 AUX2 inverter
generates -7.5V for negative CCD bias. The AUX3 con-
troller generates a regulated current for a series net-
work of four white LEDs that backlight the LCD.
Figure 2. Typical Operating Circuit for 2 AA Cells
Figure 2 is optimized for 2-cell AA inputs (1.5V to 3.7V)
by connecting the step-down input (PVSD) to the main
output (PVM). The main 3.3V output operates directly
from the battery as a step-up (SUSD = PVSD). The 1.8V
core output now operates as a boost-buck with efficien-
cy up to 90%. The rest of the circuit is unchanged from
Figure 1.
Figure 3. Typical Operating Circuit for 2 AA Cells
and 1-Cell Li+
The MAX1566/MAX1567 can also allow either 1-cell Li+
or 2 AA cells to power the same design. If the step-
down and main inputs are both connected to PVSU,
then both the 3.3V and 1.8V outputs operate as boost-
buck converters. There is an efficiency penalty com-
pared to stepping down VSD directly from the battery,
but that is not possible with a 1.5V input.
Furthermore, the cascaded boost-buck efficiency com-
pares favorably with other boost-buck techniques.
LED, LCD, and Other Boost Applications
Any AUX channel (except for the AUX2 inverter on the
MAX1567) can be used for a wide variety of step-up
applications. These include generating 5V or some
other voltage for motor or actuator drive, generating
15V or a similar voltage for LCD bias, or generating a
step-up current source to efficiently drive a series array
of white LEDs to display backlighting. Figures 7 and 8
show examples of these applications.
Multiple-Output Flyback Circuits
Some applications require multiple voltages from a sin-
gle converter channel. This is often the case when gen-
erating voltages for CCD bias or LCD power. Figure 9
shows a two-output flyback configuration with an AUX
channel. The controller drives an external MOSFET that
switches the transformer primary. Two transformer sec-
ondaries generate the output voltages. Only one posi-
tive output voltage can be fed back, so the other
voltages are set by the turns-ratio of the transformer
secondaries. The load stability of the other secondary
voltages depends on transformer leakage inductance
and winding resistance. Voltage regulation is best
when the load on the secondary that is not fed back is
small compared to the load on the one that is fed back.
Regulation also improves if the load-current range is
limited. Consult the transformer manufacturer for the
proper design for a given application.
Transformerless Inverter for Negative CCD
Bias (AUX2, MAX1567)
On the MAX1567, AUX2 is set up to drive an external P-
channel MOSFET in an inverting configuration. DL2 drives
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 negative CCD
load current. Figure 10 shows an example circuit.
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 11. 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.
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