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LT3436

3436fa

APPLICATIONS INFORMATION

FB RESISTOR NETWORK

The suggested resistance (R2) from FB to ground is 10k

1%. This reduces the contribution of FB input bias current

to output voltage to less than 0.2%. The formula for the

resistor (R1) from VOUT to FB is:

OUT

21 2

2 0 2

−

()

−

.( .

)

Figure 2. Feedback Network

OUTPUT CAPACITOR

Step-up regulators supply current to the output in pulses.

The rise and fall times of these pulses are very fast. The

output capacitor is required to reduce the voltage ripple

this causes. The RMS ripple current can be calculated

from:

RIPPLE RMS

OUT

() =

−

() /

The LT3436 will operate with both ceramic and tantalum

output capacitors. Ceramic capacitors are generally cho-

sen for their small size, very low ESR (effective series

resistance), and good high frequency operation, reducing

output ripple voltage. Their low ESR removes a useful zero

in the loop frequency response, common to tantalum

capacitors. To compensate for this, the VC loop compen-

sation pole frequency must typically be reduced by a factor

of 10. Typical ceramic output capacitors are in the 4.7

µF

–

1.2V

VSW

GND

3436 F02

10k

OUTPUT

ERROR

AMPLIFIER

LT3436

to 22

µF range. Since the absolute value of capacitance

defines the pole frequency of the output stage, an X7R or

X5R type ceramic, which have good temperature stability,

is recommended.

Tantalum capacitors are usually chosen for their bulk

capacitance properties, useful in high transient load appli-

cations. ESR rather than absolute value defines output

ripple at 800kHz. Values in the 22

µF to 100µF range are

generally needed to minimize ESR and meet ripple current

ratings. Care should be taken to ensure the ripple ratings

are not exceeded.

Table 1. Surface Mount Solid Tantalum Capacitor ESR and

Ripple Current

E Case Size

ESR (Max,

Ω)

Ripple Current (A)

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

D Case Size

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

C Case Size

AVX TPS

0.2 (typ)

0.5 (typ)

INPUT CAPACITOR

Unlike the output capacitor, RMS ripple current in the

input capacitor is normally low enough that ripple current

rating is not an issue. The current waveform is triangular,

with an RMS value given by:

Lf V

RIPPLE RMS

OUT

() =

()

−

()

()( )(

)

029

At higher switching frequency, the energy storage require-

ment of the input capacitor is reduced so values in the

range of 2.2

µF to 10µF are suitable for most applications.

Y5V or similar type ceramics can be used since the

absolute value of capacitance is less important and has no

significant effect on loop stability. If operation is required

close to the minimum input voltage required by either the

output or the LT3436, a larger value may be necessary.

This is to prevent excessive ripple causing dips below the

minimum operating voltage resulting in erratic operation.

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