MAX1729 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX1729
Maxim Integrated
MAX1729 Datasheet PDF : 12 Pages
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ECB and LCD Display Bias Supply with Accurate
Output Voltage and Temperature Compensation
2) Given the maximum output voltage (VMAX) and mini-
mum output voltage (VMIN), calculate values for R3 and
R4 as follows:
R3 = 1/2
R1
VFB
VMAX – VMIN
R4 = R3
3) For first-order temperature compensation, calculate
R5 as shown below. (If temperature compensation is
not used, leave R5 open.)
R5
=
R1
Tempco
16.5mV/°C
where Tempco is the negative temperature coefficient
needed to compensate the ECB or LCD display for
changes in temperature.
4) Solve for VCOMP. The duty cycle used here corre-
sponds to the duty cycle that yields the maximum out-
put voltage, not including first-order temperature
compensation.
VCOMP
=
VFB
1
–
Duty Cycle ⋅
R4
R3 + R4
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.)
1
R2
=
1
VFB
VOUT
R1
+
VCOMP
R3
+
VFB
R5
−
1
R1
+
1
R3
+
1
R5
External Component Value Example
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) VMAX = 10V and IFB = 29.24µA is within the limits
and yields a reasonable resistor value, therefore:
R1 =
10V − 1.228V
29.24µA
= 300kΩ
2) VMAX = 10V and VMIN = 5V, therefore:
R3
=
1/2
300kΩ
5V
1.228
=
36,840Ω
with R3 = 36.7kΩ, then VMIN = 5.019V. Let R4 =
R3 = 36.7kΩ.
3) Tempco = 33mV/°C, therefore:
R5
=
300kΩ
33mV/°C
16.5mV / °C
=
150kΩ
4) If external circuitry limits the duty cycle to 90%, the
following equation is true:
VCOMP
= 1.228 1 −
0.9
2
= 0.6754V
5) Solving for R2:
1
R2
=
VOUT
R1
+
VCOMP
R3
+
VFB
R5
1
VFB
−
1
R1
+
1
R3
+
1
R5
=
1
56560
With R2 = 56kΩ, a duty cycle of 87.4% generates a
VOUT of 10V.
Component Selection
Inductors
Use a 220µH inductor to maximize output current
(2.5mA typical). Use an inductor with DC resistance
less than 10Ω and a saturation current exceeding
35mA. For lower peak inductor current, use a 470µH
inductor with DC resistance less than 20Ω and a satu-
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.
Capacitors
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.
_______________________________________________________________________________________ 9
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