Rainbow Electronics MAX1729 User Manual
Page 9
2) Given the maximum output voltage (V
MAX
) and mini-
mum output voltage (V
MIN
), calculate values for R3 and
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
COMP
. The duty cycle used here corre-
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.)
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) V
MAX
= 10V and I
FB
= 29.24µA is within the limits
and yields a reasonable resistor value, therefore:
2) V
MAX
= 10V and V
MIN
= 5V, therefore:
with R3 = 36.7k
Ω
, then V
MIN
= 5.019V. Let R4 =
R3 = 36.7k
Ω
.
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
Ω
, a duty cycle of 87.4% generates a
V
OUT
of 10V.
Component Selection
Inductors
Use a 220µH inductor to maximize output current
(2.5mA typical). Use an inductor with DC resistance
less than 10
Ω
and a saturation current exceeding
35mA. For lower peak inductor current, use a 470µH
inductor with DC resistance less than 20
Ω
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.
1
R2
V
R1
V
R3
V
R5
1
V
1
R1
1
R3
1
R5
1
56560
OUT
COMP
FB
FB
=
+
+
−
+
+
=
V
1.228 1
0.9
2
0.6754V
COMP
=
−
=
R5
300k
33mV/ C
=
°
°
=
Ω
Ω
/
16. mV
k
5
150
C
R3
1/2
300k
5V
1.228
36,840
=
=
Ω
Ω
R1
10V 1.228V
29.24 A
=
−
µ
=
300k
Ω
1
R2
1
V
V
R1
V
R3
V
R5
1
R1
1
R3
1
R5
FB
OUT
COMP
FB
=
+
+
−
+
+
V
=V
1 – Duty Cycle
R4
R3
R4
COMP
FB
⋅
+
R5 =
R1
Tempco
16.5mV/ C
°
R3 = 1/2
R1
V
– V
V
R4 = R3
MAX
MIN
FB
MAX1729
ECB and LCD Display Bias Supply with Accurate
Output Voltage and Temperature Compensation
_______________________________________________________________________________________
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