MAX1800EVKIT Maxim Integrated, MAX1800EVKIT Datasheet - Page 4

• Manufacturer Part Number: MAX1800EVKIT
• Description: Power Management IC Development Tools
• Manufacturer: Maxim Integrated
• Series: MAX1800r

MAIN is set by a voltage divider, which drops the out-
put voltage to the 1.25V feedback threshold voltage. To
change the 3.3V setting of MAIN, change the resistor
divider ratio by changing R21. Use:
The MAIN output voltage may be set to any voltage
between 2.7V and 5.5V. For additional information, see
the MAX1800 data sheet.
The LDO output voltage is set by a voltage divider,
which drops the output voltage to the 1.25V feedback
threshold voltage. To change the 1.8V setting of LDO,
change the resistor divider ratio by changing R18. Use:
The LDO output voltage may be set to any voltage
between 1.25V and 5.5V but must remain less than the
voltage at MAIN when powered from MAIN. The
MAX1800 EV kit is configured so that MAIN powers the
LDO input; however, opening up the short across R33
and shorting R34 will allow LDO to be powered from IN.
A flyback circuit generates OUT1_ and OUT2_. This
allows multiple, positive, or negative voltages to be
generated by a single converter and allows the volt-
ages to drop to 0V when the converter is disabled. The
transformer must be designed for a given set of output
voltages.
On the flyback circuits, only a single output voltage is
used to regulate all the voltages. All other voltages are
controlled by the turns-ratio of the transformer. If anoth-
er set of output voltages is required, a transformer with
a different secondary turns-ratio must be used. Consult
MAX1800 Evaluation Kit
Table 2. Flyback Converter Feedback
Resistors
4
_______________________________________________________________________________________
OUTPUT
OUT_A
OUT_B
OUT_C
Setting the Flyback Circuit Voltages
R21 = 80kΩ / V
R18 = 80kΩ / V
Setting the Main Output Voltage
Setting the LDO Output Voltage
OUT1_
R13
R11
R12
✕
✕
( V
( V
MAX1800 EV Kit
Customizing the
(OUT1_ and OUT2_)
MAIN
LDO
- 1.25V )
- 1.25V )
OUT2_
R14
R16
R15
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, the resistor value (Table 2) is:
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:
For OUT4, choose:
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 EV kit has DCON_
shorted to REF, producing a default duty cycle of 84%.
See the MAX1800 data sheet for additional information.
All of the switching regulators are synchronized to a
single oscillator frequency. The oscillator capacitor
(C2) is charged through R1 and discharged internally
by the MAX1800. The EV kit is designed to operate at
440kHz with MAIN set to 3.3V. If a different MAIN volt-
age is used, or if a different oscillator frequency is
desired, change resistor R1 or capacitor C2. Consult
the MAX1800 data sheet for the correct values to use
for R1 and C2.
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/OFF jumper for that output, and place the con-
trol signal on the corresponding ON_ pad. To enable
the output, make sure that the voltage of the control
signal at the corresponding ON_ pad is >1.6V. To dis-
able it, make sure the voltage at the corresponding
ON_ pad is <0.3V.
Setting the Step-Up Output Voltage (OUT3
Logic Control of the Output Voltages
R24 = 80kΩ / V
R4 = 80kΩ / V
R = 80kΩ / V
Setting the Maximum Duty Cycle
Setting the Switching Frequency
✕
✕
✕
( V
( V
( V
OUT
OUT3
OUT4
- 1.25V )
- 1.25V )
- 1.25V )
and OUT4)