LTM4677 datasheet(49/110 Pages) AD

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LTC3888-1

Rev. 0

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surface mount applicability; and electrolytic capacitors

have higher ESR and can dry out. Sanyo OS-CON SVP(D)

series, Sanyo POSCAP TQC series, or Panasonic EE-FT

series aluminum electrolytic capacitors can be used in

parallel with high performance ceramic capacitors as

an effective means of achieving low ESR and high bulk

capacitance.

In addition to PWM bulk input capacitance, a small

(0.01μF to 1μF) bypass capacitor between the chip VIN

pin and ground, placed close to the LTC3888-1, is also

suggested. A small resistor placed between the bulk CIN

and the VIN pin/bypass provides further isolation between

rails. However, if the time constant of any such R-C net-

work on the VIN pin exceeds 30ns, dynamic line transient

response can be adversely affected.

COUT SELECTION

The selection of COUT is primarily determined by the ESR

required to minimize voltage ripple and load step tran-

sients. The output ripple ΔVOUT is approximately bounded

by:

ΔVOUT ≤ ΔIL ESR +

8 • f PWM •COUT

⎛

⎝

⎜

⎞

⎠

⎟

where ΔIL is the inductor ripple current.

ΔIL =

VOUT

L • f PWM

1–

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

Since ΔIL increases with input voltage, the output ripple

voltage is highest at maximum input voltage. Typically

once the ESR requirement is satisfied, the capacitance is

adequate for filtering and has the necessary RMS current

rating.

Manufacturers such as Sanyo, Panasonic and Cornell

Dubilier should be considered for high performance

through-hole capacitors. The OS-CON semiconductor

electrolyte capacitor available from Sanyo has a good

(ESR)(size) product. Additional ceramic capacitors in par-

allel with polarized capacitors is recommended to offset

the effect of lead inductance.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or transient current

handling requirements of the application. Aluminum

electrolytic and dry tantalum capacitors are both avail-

able in surface mount configurations. New polymer sur-

face mount capacitors also offer very low ESR but have

much lower capacitive density. In the case of tantalum,

it is critical that the capacitors are surge tested for use

in switching power supplies. Several excellent output

capacitor choices include the Sanyo POSCAP TPD/E/F

series, the Kemet T520, T530 and A700 series, NEC/Tokin

NeoCapacitors and Panasonic SP series. Other suitable

capacitor types include Nichicon PL series and Sprague

595D series. Consult the manufacturer for other specific

recommendations.

PROGRAMMABLE LOOP COMPENSATION

Because the LTC3888-1 uses an OTA error amplifier

architecture, Type II compensation is most commonly

applied for stabilizing the voltage control loop as shown

in Figure 25. The LTC3888-1 offers programmable loop

compensation to optimize the transient response without

requiring a hardware change. Internal error amplifier gm

can be varied from 1mmho to 5.73 mmho, and internal

compensation resistor RITH can be varied from 1kΩ to

62kΩ with the MFR_PWM_COMP command. A maximum

of two external capacitors are then required to stabilize

each voltage control loop.

By adjusting EA gm and RITH the LTC3888-1 can pro-

vide flexible Type II compensation for loop optimization

over a wide range of output capacitance. Adjusting gm

will change compensation gain over the entire frequency

range without adjusting any pole-zero locations as shown

in Figure 26.

APPLICATIONS INFORMATION

Figure 25. Programmable Loop Compensation

–

VREF

388812 F25

CTHP

CTH

ITH

RITH

ITHR

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