MAX1802EVKIT データシートの表示（PDF） - Maxim Integrated

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MAX1802EVKIT

MAX1802 Evaluation Kit for Digital Still Cameras

Maxim Integrated 

MAX1802 Evaluation Kit for

Digital Still Cameras

Setting the Flyback Circuit Voltages

(OUT1_ and OUT2_)

A flyback circuit generates OUT1_ and OUT2_. This

allows multiple positive or negative voltages to be gen-

erated by a single converter and allows the voltages to

drop to 0 when the converter is disabled. The trans-

former must be designed for a given set of output voltages.

On the flyback circuits, only a single output voltage is

directly regulated by the MAX1802. All other voltages

are controlled by the transformer’s turns-ratio. If another

set of output voltages is required, a transformer with a

different secondary turns-ratio must be used. Consult

the transformer manufacturer for details. Typically, the

highest power, positive voltage output of the flyback

transformer is fed back to the controller. To change the

regulated output voltage, use a different resistor in the

voltage-divider. Table 2 lists the resistors used for each

output. Leave the resistors off (open) for unregulated

outputs.

For a given output voltage, if the bottom resistor is

100kΩ, then the top resistor value is:

R__ = 80kΩ/V x (VOUT - 1.248V)

Setting the Step-Up Output Voltage

(OUT3 and OUT4)

The output voltage of the step-up circuit voltages

(OUT3 and OUT4) may be set to any voltage above

+1.25V. Note that if the battery voltage is greater than

the step-up regulation voltage, the output voltage will

rise above the regulation voltage. To set the output volt-

age, choose the voltage-divider resistors. For OUT3,

choose:

R18 = 80kΩ/V x (VOUT3 - 1.248V)

For OUT4, choose:

R2 = 80kΩ/V x (VOUT3 - 1.248V)

Setting the Maximum Duty Cycle

DCON1, DCON2, and DCON3 set the maximum duty

cycle for controllers 1, 2, and 3, respectively. A resistor-

divider from REF to DCON_ sets the corresponding

maximum duty cycle up to 90%. The MAX1802 EV kit

has DCON_ shorted to REF, producing a default duty

cycle of 76%. Refer to the MAX1802 IC data sheet for

additional information.

Setting the Switching Frequency

All of the switching regulators are synchronized to a

single oscillator frequency. The oscillator capacitor

(C1) is charged through R1 and discharged internally

by the MAX1802. The MAX1802 EV kit operates at

400kHz, with MAIN set to 3.3V. Change resistor R1 or

capacitor C1 if a different MAIN voltage is used or

another frequency is desired. Refer to the MAX1802 IC

data sheet for the correct values to use for R1 and C1.

Logic Control of the Output Voltages

Each controller may be independently turned off or on

using jumpers (JU1–JU6) or with logic voltages. To

control an output using an external logic signal, remove

the ON and OFF jumper for that output and place the

control signal on the corresponding ON_ pad. To

enable MAIN, drive the ON pad greater than +1.8V. To

enable CORE or OUT1, drive the corresponding ON_

pad greater than +1.6V. To enable OUT2, OUT3, or

OUT4 with the default maximum duty cycle, drive the

corresponding ON_ pad to greater than or equal to the

reference voltage. If a resistor-divider is used for duty

cycle adjustment (R5, R6, R9, R10, R11, and R12), use

an open-drain driver on the corresponding ON_ pad.

See the Setting the Maximum Duty Cycle section for

additional information. To disable each output individu-

ally, drive the voltage at the corresponding ON_ pad

less than +0.3V.

Note that the voltages at OUT3 and OUT4 will drop

slightly below the battery voltage when off, due to the

DC current path through the inductor and output rectifi-

er. For the step-up outputs, a switch may be added

between the output voltage and the load to disconnect

them while the output is disabled.

Table 2. Flyback Converter Feedback

Resistors

OUTPUT

OUT_A

OUT_B

OUT_C

OUT1_

R30

R29

R31

OUT2_

R26

R25

R27

4 _______________________________________________________________________________________

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