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.

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.

and yields a reasonable resistor value, therefore:

with R3 = 36.7k

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

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

= 300kΩ

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













ECB and LCD Display Bias Supply with Accurate

Output Voltage and Temperature Compensation

_______________________________________________________________________________________

9