LTC3822 datasheet(13/20 Pages) LINER

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LTC3822

3822f

LTC3822

VFB

VOUT

CFF

3822 F03

CIN and COUT Selection

In continuous mode, the source current of the top MOS-

FET is a square wave of duty cycle (VOUT/VIN). To prevent

large voltage transients, a low ESR input capacitor sized

for the maximum RMS current must be used. The maxi-

mum RMS capacitor current is given by:

CINRe

•

•–

quiredI

RMS

MAX

OUT

≈

()12

This formula has a maximum value at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of life.

This makes it advisable to further derate the capacitor or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet the

size or height requirements in the design. Due to the high

operating frequency of the LTC3822, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

The selection of COUT is driven by the effective series

resistance (ESR). Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering. The

output ripple (

∆VOUT) is approximated by:

∆≈

⎛

⎝⎜

⎞

⎠⎟

ESR

OUT

RIPPLE

OUT

•

••

where f is the operating frequency, COUT is the output

capacitance and IRIPPLE is the ripple current in the induc-

tor. The output ripple is highest at maximum input voltage

since IRIPPLE increase with input voltage.

Top-Side MOSFET Drive Supply (CB, DB)

In the Functional Diagram, external bootstrap capacitor CB

is charged from a boost power source (usually VIN)

through diode DB when the SW node is low. When a

MOSFET is to be turned on, the CB voltage is applied

across the gate-source of the desired device. When the

top-side MOSFET is on, the BOOST pin voltage is above the

input supply. VBOOST = 2VIN. CB must be 100 times the

total input capacitance of the top-side MOSFET. The

reverse breakdown of DB must be greater than VIN(MAX).

Figure 6 shows how a 5V gate drive can be achieved if a

secondary 5V supply is available. Note that in applications

where the supply voltage to CB exceeds VIN, the BOOST pin

will draw approximately 500

µA in shutdown mode.

Setting Output Voltage

The LTC3822 output voltage is set by an external feedback

resistor divider carefully placed across the output, as

shown in Figure 3. The regulated output voltage is deter-

mined by:

OUT

⎛

⎝⎜

⎞

⎠⎟

.•

For most applications, a 59k resistor is suggested for RA.

In applications where minimizing the quiescent current is

critical, RA should be made bigger to limit the feedback

divider current. If RB then results in very high impedance,

it may be beneficial to bypass RB with a 50pF to 100pF

capacitor CFF.

APPLICATIO S I FOR ATIO

Figure 3. Setting the Output Voltage

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