MAX1817EUB Maxim Integrated Products, MAX1817EUB Datasheet - Page 9

• Manufacturer Part Number: MAX1817EUB
• Description: IC CONV DUAL DCDC STEP 10-UMAX
• Manufacturer: Maxim Integrated Products
• Type: Step-Up (Boost)r

Specifications of MAX1817EUB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: 3.3V or Adj Main, -28 ~ 28 V or Adj Secondary
• Current - Output: 125mA
• Frequency - Switching: 500kHz
• Voltage - Input: 1.5 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Power - Output: 444mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Figure 2. Setting Main Output Voltage Using External Resistors
The MAX1817’s internal circuitry is powered from OUT.
The main step-up converter has a low-voltage startup
circuit to control main DC-DC converter operation until
V
threshold. The minimum startup voltage is a function of
load current (see Typical Operating Characteristics).
The MAX1817 main converter typically starts up into a
35Ω load with input voltages down to +1.5V, allowing
startup with two alkaline cells even in deep discharge.
The MAX1817 features independent shutdown control
of the main and LCD step-up converters. With both
converters shut down, supply current is reduced to
0.1µA. A logic low at ON shuts down the main step-up
converter, and LX enters a high-impedance state.
However, the main output remains connected to the
input through the inductor and output rectifier, holding
V
the main converter is shut down. If the input voltage is
sufficiently high to drive V
lockout voltage, the LCD step-up converter operates.
A logic low at ONLCD shuts down the LCD step-up
converter, and LXLCD enters a high-impedance state.
The LCD output remains connected to the input
through the inductor and output rectifier, holding it to
one diode drop below the input.
OUT
OUT
OFF
OFF
V
IN
to one diode drop below the input voltage when
exceeds the 2.2V (typ) undervoltage lockout
MAIN
LCD
ON
ON
C1
10µF
ONLCD
ON
AGND
LX
L2
10µH
MAX1817
_______________________________________________________________________________________
LXLCD
GND
FBLCD
Shutdown: ON and ONLCD
Compact, High-Efficiency, Dual-Output
OUT
L1
10µH
FB
OUT
D2
D1
Low-Voltage Startup
R3
300k
R4
100k
4.7pF
above the undervoltage
C4
C3
22µF
R1
1M
MAIN 5V
R2 75k
C2
1µF
LCD
18V
Step-Up DC-DC Converter
___________________Design Procedure
The main step-up converter feedback input (FB) fea-
tures Dual Mode operation. With FB grounded, the
main output voltage is preset to 3.3V. It can also be
adjusted from 2.5V to 5.5V with external resistors R3
and R4 as shown in Figure 2. To set the output voltage
externally, select resistor R4 from 10kΩ to 100kΩ.
Calculate R3 using:
where V
5.5V.
Set the LCD output voltage with two external resistors
R1 and R2 as shown in Figure 3. Since the input leak-
age current at FBLCD has a maximum of 50nA, large
resistors can be used without significant accuracy loss.
Begin by selecting R2 in the 10kΩ to 100kΩ range, and
calculate R1 using the following equation:
where V
to 28V.
Figure 3. Typical Application Circuit
OFF
OFF
V
IN
MAIN
LCD
FB
FBLCD
ON
ON
= 1.25V, and V
R1 = R2 [(V
Setting the Main Output Voltage
C1
10µF
R3 = R4 [(V
Setting the LCD Output Voltage
= 1.25V, and V
ONLCD
ON
AGND
LX
L2
10µH
MAX1817
LXLCD
FBLCD
GND
LCD
OUT
L1
10µH
FB
OUT
OUT
D2
/ V
D1
4.7pF
/ V
LCD
FBLCD
can range from 2.5V to
C4
FB
can range from V
) – 1]
C3
22µF
) – 1]
R1
1M
MAIN 3.3V
R2 75k
C2
1µF
LCD 18V
IN
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