MAX1729 Maxim, MAX1729 Datasheet - Page 9

• Manufacturer Part Number: MAX1729
• Description: ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
• Manufacturer: Maxim

2) Given the maximum output voltage (V
mum output voltage (V
R4 as follows:
3) For first-order temperature compensation, calculate
R5 as shown below. (If temperature compensation is
not used, leave R5 open.)
where Tempco is the negative temperature coefficient
needed to compensate the ECB or LCD display for
changes in temperature.
4) Solve for V
sponds to the duty cycle that yields the maximum out-
put voltage, not including first-order temperature
compensation.
where a 90% duty cycle corresponds to Duty Cycle = 0.9.
5) Use the results from the above calculations to solve
for R2. (For applications not utilizing temperature com-
pensation, use 1 / R5 = 0.)
The example application requires the output voltage to
adjust between 5V and 10V, using the circuit shown in
Figure 3. The device in our example needs a tempera-
ture coefficient of 33mV/°C, which yields the following
results.
1) V
and yields a reasonable resistor value, therefore:
ECB and LCD Display Bias Supply with Accurate
Output Voltage and Temperature Compensation
MAX
V
COMP
= 10V and I
R2
R1
1
R3 = 1/2
R4 = R3
R5 =
=V
COMP
R1
1
FB
External Component Value Example
_______________________________________________________________________________________
V
10V 1.228V
1
FB
1 – Duty Cycle
. The duty cycle used here corre-
R3
Tempco
29.24 A
1
FB
MIN
R1
V
V
MAX
= 29.24µA is within the limits
OUT
R1
R5
), calculate values for R3 and
1
R1
– V
16.5mV/ C
V
COMP
R3
MIN
300k
R3
V
MAX
V
R5
FB
FB
R4
) and mini-
R4
2) V
with R3 = 36.7k , then V
R3 = 36.7k .
3) Tempco = 33mV/°C, therefore:
4) If external circuitry limits the duty cycle to 90%, the
following equation is true:
5) Solving for R2:
With R2 = 56k , a duty cycle of 87.4% generates a
V
Use a 220µH inductor to maximize output current
(2.5mA typical). Use an inductor with DC resistance
less than 10
35mA. For lower peak inductor current, use a 470µH
inductor with DC resistance less than 20
ration current over 18mA. This limits output current to
typically less than 1mA. See Table 1 for a list of recom-
mended inductors. The inductor should be connected
from the battery to the LX pin, as close to the IC as pos-
sible.
The equivalent series resistance (ESR) of output capac-
itor C2 directly affects output ripple. To minimize output
ripple, use a low-ESR capacitor. A physically smaller
capacitor, such as a common ceramic capacitor, mini-
mizes board space and cost while creating an output
ripple that’s acceptable in most applications. Refer to
Table 2 for recommended capacitor values.
OUT
MAX
of 10V.
R5
V
R3
= 10V and V
COMP
R2
1
R1
1
1/2
33mV/ C
300k
and a saturation current exceeding
R3
V
1
1.228 1
OUT
R1
300k
MIN
5V
R5
MIN
1
= 5V, therefore:
16. mV
V
COMP
Component Selection
= 5.019V. Let R4 =
1.228
R3
5
0.9
56560
2
/
1
C
V
R5
FB
36,840
0.6754V
150
V
FB
1
and a satu-
Capacitors
Inductors
k
9