LT3437IDD datasheet(13/28 Pages) LINER

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LT3437

3437fc

is high enough at the switching frequency, output ripple

voltage (although smaller for a ceramic output capacitor)

may still affect the proper operation of the regulator. A

filter capacitor CF in parallel with the RC/CC network, along

with a small feedforward capacitor CFB, is suggested to

control possible ripple at the VC pin. The LT3437 can be

stabilized using a 100

µF ceramic output capacitor and VC

component values of CC = 1500pF, RC = 25k, CF = 330pF

and CFB =27pF.

OUTPUT RIPPLE VOLTAGE

Figure 3 shows a typical output ripple voltage waveform

for the LT3437. Ripple voltage is determined by the

impedance of the output capacitor and ripple current

through the inductor. Peak-to-peak ripple current through

the inductor into the output capacitor is:

VL f

OUT

P-P =

()

()( )( )

–

For high frequency switchers the ripple current slew rate

is also relevant and can be calculated from:

Peak-to-peak output ripple voltage is the sum of a triwave

created by peak-to-peak ripple current times ESR and a

square wave created by parasitic inductance (ESL) and

ripple current slew rate. Capacitive reactance is assumed

to be small compared to ESR or ESL.

ESR

ESL

RIPPLE =

()( )+()

P-P

Example: with VIN = 12V, VOUT = 3.3V, L = 100µH, ESR =

0.075

Ω, ESL = 10nH:

IP-P =

()(

)

()

−

()(

)

33 12 33

12 100

6 200 3

0 120

100

012 6

.– .

–

VRIPPLE = (0.120A)(0.075) + (10e – 9)(0.12e6)

= 0.009 + 0.0012 = 10.2mVP-P

voltage could be unacceptable. Table 2 shows some

typical solid tantalum surface mount capacitors.

Table 2. Surface Mount Solid Tantalum Capacitor ESR

and Ripple Current

E CASE SIZE

ESR MAX (

Ω)

RIPPLE CURRENT (A)

AVX TPS

0.1 to 0.3

0.7 to 1.1

D CASE SIZE

AVX TPS

0.1 to 0.3

0.7 to 1.1

C CASE SIZE

AVX TPS

0.2

0.5

Many engineers have heard that solid tantalum capacitors

are prone to failure if they undergo high surge currents.

This is historically true, and type TPS capacitors are

specially tested for surge capability, but surge ruggedness

is not a critical issue with the output capacitor. Solid

tantalum capacitors fail during very high turn-on surges

which do not occur at the output of regulators. High

discharge surges, such as when the regulator output is

dead shorted, do not harm the capacitors.

Unlike the input capacitor RMS, ripple current in the

output capacitor is normally low enough that ripple cur-

rent rating is not an issue. The current waveform is

triangular with a typical value of 30mARMS. The formula to

calculate this is:

Output capacitor ripple current (RMS)

Lf V

RIPPLE RMS

OUT

()

.–

()(

)

()( )( )

029

IP-P

CERAMIC CAPACITORS

Higher value, lower cost ceramic capacitors are now

becoming available. They are generally chosen for their

good high frequency operation, small size and very low

ESR (effective series resistance). Low ESR reduces output

ripple voltage but also removes a useful zero in the loop

frequency response, common to tantalum capacitors. To

compensate for this, a resistor RC can be placed in series

with the VC compensation capacitor CC (Figure 10). Care

must be taken, however, since this resistor sets the high

frequency gain of the error amplifier, including the gain at

the switching frequency. If the gain of the error amplifier

APPLICATIO S I FOR ATIO

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