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MAX685EEE
Dual-Output Positive and Negative / DC-DC Converter for CCD and LCD
General DescriptionThe MAX685 DC-DC converter provides low-noise dual
outputs for powering CCD imaging devices and LCDs.
This device uses a single inductor to provide indepen-
dently regulated positive and negative outputs.
Integrated power switches are included in the small
16-pin QSOP package (same size as an 8-pin SO) to
save space and reduce cost.
Each output delivers up to 10mA from a +2.7V to +5.5V
input voltage range. Output voltages are set indepen-
dently up to 24V and down to -9V. With a few additional
low-cost components, the output voltages can be set at
up to 45V and down to -16V. Output ripple magnitude is
30mVp-p. The MAX685 uses a fixed-frequency, pulse-
width-modulated (PWM) control scheme at 220kHz or
400kHz to permit output noise filtering and to reduce
the size of external components. The frequency can
also be synchronized to an external clock signal
between 200kHz and 480kHz.
The MAX685 has a power-OK indicator output (POK)
that signals when both outputs are within regulation. A
logic-controlled shutdown completely turns off both out-
puts and reduces supply current to 0.1µA. The user
can also set which output turns on first.
The preassembled MAX685 evaluation kit is available to
reduce design time.
ApplicationsCamcordersLCDs
Digital CamerasCCD Imaging Devices
Notebooks
FeaturesDual Output Using a Single InductorLow-Noise Output, 30mVp-p RippleOutput Voltages up to 24V and down to -9V
(up to 45V and down to -16V with added
components)Internal Switches in a Small Package220kHz/400kHz Fixed-Frequency PWM OperationFrequency Can Be Synchronized to
External ClockPower-OK IndicatorSelectable Power-On Sequencing0.1µA Logic-Controlled Shutdown
MAX685
DC-DC Converter for CCD and LCDypical Operating Circuit
Pin Configuration
Ordering Information
MAX685
Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VDD= VP = 5V, TA
= 0°C to +85°C unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VDD, VP to GND........................................................-0.3V to +6V
PGND to GND.......................................................-0.3V to +0.3V
VDDto VP...............................................................-0.3V to +0.3V
LXN, POK to GND..................................................-0.3V to +30V
LXP to VDD..............................................................-15V to +0.3V
REF, SEQ, SHDNto GND...........................-0.3V to (VDD+ 0.3V)
FBP, FBN, SYNC to GND.........................................-0.3V to +6V
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)............667mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +165°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX685
Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
ELECTRICAL CHARACTERISTICS (continued)(VDD= VP = 5V, TA
= 0°C to +85°C unless otherwise noted. Typical values are at TA= +25°C.)
ELECTRICAL CHARACTERISTICS (VDD, VP = 5V, TA
= -40°C to +85°C unless otherwise noted.) (Note 2)
Note 1:Negative output voltage can be larger magnitude for lower values of VDD. The voltage between VDDand VOUT-must not
exceed 14.5V.
Note 2:Specifications to -40°C are guaranteed by design, not production tested.
MAX685
Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
Typical Operating Characteristics(Circuit of Figure 3, VOUT+= 15V, VOUT- = -7.5V, TA= +25°C, unless otherwise noted.)
MAX685
Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
Typical Operating Characteristics (continued)(Circuit of Figure 3, VOUT+= 15V, VOUT- = -7.5V, TA= +25°C, unless otherwise noted.)
_______________Detailed DescriptionThe MAX685 DC-DC converter accepts an input voltage
between +2.7V and +5.5V and generates both a positive
and negative voltage, using a single inductor (Figure 1).
It alternates between acting as a step-up converter and
as an inverting converter on a cycle-by-cycle basis. Both
output voltages are independently regulated.
Each output is separately controlled by a pulse-width-
modulated (PWM) current mode regulator. This allows
the part to operate at a fixed frequency for use in noise-
sensitive applications. An internal oscillator runs at
220kHz or 400kHz, or can be synchronized to an exter-
nal signal. Since switching alternates between the two
regulators, each operates at half the oscillator frequency
(110kHz, 200kHz, or half the sync frequency). The oscil-
lator can be synchronized to a 200kHz to 480kHz clock.
On the first cycle of operation, the part operates as a
step-up converter. LXP connects to VDD, LXN pulls to
ground, and the inductor current rises. Once the induc-
tor current rises to a level set by the positive-side error
amplifier, LXN releases and the inductor current flows
through D2 to the positive output. When the inductor
current drops to zero (which happens each cycle under
normal, discontinuous operation), LXN returns to the
input voltage.
On the second cycle, LXN is held at ground. LXP is
pulled up to the input voltage until the current reaches
the limit set by the negative error amplifier. Then LXP is
released and the inductor current flows through D1 to
the negative output. Once the inductor current reaches
zero, the voltage at LXP returns to ground. The wave-
forms at LXN and LXP are shown in Figure 2 for a typi-
cal pair of cycles.
The current into the LXN pin is sensed to measure the
inductor current. The MAX685 controls the inductor cur-
rent to regulate both the positive and negative output
voltages.
MAX685
Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
Pin Description
