MAX1817EUB Maxim Integrated Products, MAX1817EUB Datasheet - Page 10

• 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

The MAX1817 can generate a negative LCD output by
adding a diode-capacitor charge-pump circuit (D3, D4,
and C6) to the LXLCD pin as shown in Figure 4. FBLCD
is driven through a resistive voltage-divider from the
positive output, which is not loaded, allowing a very
small capacitor value at C2. For best stability and low-
est ripple, the time constant of the R1 + R2 series com-
bination and C2 should be near that of C5 and the
effective load resistance. Output load regulation of the
negative output degrades compared to the standard
positive output circuit and may rise at very light loads. If
this is not acceptable, reduce the resistance of R1 and
R2, while maintaining their ratio, to effectively preload
the output with a few hundred µA. This is why the R1
and R2 values shown in Figure 4 are lower than typical
values for a positive-output design. When loaded, the
magnitude of the negative output voltage is slightly
lower (closer to ground by approximately a diode for-
ward voltage) than the voltage on C2.
The MAX1817’s high switching frequency allows the
use of small surface-mount inductors. The 10µH values
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
Figure 4. Negative Voltage for LCD Bias
10
V
IN
______________________________________________________________________________________
Using a Charge Pump to Make Negative
C1
10µF
ONLCD
ON
AGND
Applications Information
LX
MAX1817
L2
10µH
LXLCD
FBLCD
GND
L1,10µH
OUT
FB
D2
**D1 = CENTRAL SEMICONDUCTOR
240k
*D3, D4 = CENTRAL SEMICONDUCTOR
CMPD7000 DUAL
CMSD4448 (1N4148)
D1**
R1
Inductor Selection
1
R3
C3
22µF
LCD Output Voltage
0.1µF
MAIN
C6
C4
10pF
16.5k
R2
D4*
D3*
C5
1µF
-19V
V
C2
0.1µF
LCD
shown in Figure 3 are recommended for most applica-
tions, although values between 4.7µH and 47µH are
suitable. Smaller inductance values typically offer a
smaller physical size for a given series resistance,
allowing the smallest overall circuit dimensions. Larger
inductance values exhibit higher output current capa-
bility, but larger physical dimensions.
Circuits using larger inductance values may start up at
lower input voltages and exhibit less ripple, but they
may provide reduced output power. This occurs when
the inductance is sufficiently large to prevent the maxi-
mum current limit from being reached before the maxi-
mum on-time expires. The inductor’s saturation current
rating should be greater than the peak switching cur-
rent. However, it is generally acceptable to bias most
inductors into saturation by as much as 20%, although
this may slightly reduce efficiency.
For best efficiency, select inductors with resistance no
greater than the internal N-channel FET resistance in
each step-up converter.
For maximum output current, choose L such that:
where t
step-up converter) or 9µs for LCD step-up converter)
and I
main step-up converter, or 0.5A for the LCD step-up
converter). With this inductor value, the maximum output
current the main converter is able to deliver is given by:
where t
and V
Schottky diode drop (0.3V typ), and V
R
For V
value of 0.5A, and V
0.325V, the available output current that the converter
can provide is at least 90mA.
For larger inductor values, I
The high maximum switching frequency of the
MAX1817 requires a high-speed rectifier. Schottky
diodes such as the Motorola MBR0530 or the Nihon
EP05Q03L are recommended. To maintain high effi-
ciency, the average current rating of the Schottky diode
should be greater than the peak switching current. A
junction diode such as the Central Semiconductor
CMPD4448 can be used for the LCD output with little
ON
, where R
PEAK
IN
OUT
I
ON
ON
OUT(MAX)
= 1.5V and V
is the switch peak current limit (0.75A for the
is the maximum switch on-time (5µs for main
are the input and output voltages, V
/ t
OFF
ON
L < [(V
I
PEAK
is the switch on-resistance.
= (V
= 0.5
ON(MAX)
OUT
OUT
= [(V
IN
✕
✕
I
= 3.3V, with a minimum I
PEAK
+ V
IN
t
PEAK
ON
✕
D
given by (0.5)
) / I
External Rectifiers
t
/ (1 + t
- V
ON
is determined by:
PEAK
IN
) / L]
) / (V
ON
]
ON
IN
/ t
OFF
- V
✕
= I
ON
)
(0.65) =
D
PEAK
is the
), V
PEAK
IN
✕