LTC7813 datasheet(20/38 Pages) LINER

20 / 38 page

LTC3895

3895fa

For more information www.linear.com/LTC3895

Incontinuousmode,thesourcecurrentofthetopMOSFET

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

large voltage transients, a low ESR capacitor sized for the

maximumRMScurrentmustbeused.ThemaximumRMS

capacitor current is given by:

CIN Required IRMS ≈

IMAX

VIN

(VOUT)(VIN − VOUT)

[

]

1/2

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

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

used for design because even significant deviations do not

offermuchrelief.Notethatcapacitormanufacturers’ripple

current ratings are often based on only 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

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

operating frequency of the LTC3895, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

A small (0.1μF to 1μF) bypass capacitor between the chip

VIN pin and ground, placed close to the LTC3895, is also

suggested. A small (≤10Ω) resistor placed between CIN

(C1) and the VIN pin provides further isolation.

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:

ΔVOUT ≈ ΔIL ESR+

8 • f • COUT

⎛

⎝

⎜

⎞

⎠

⎟

where f is the operating frequency, COUT is the output

capacitance and ∆IL is the ripple current in the inductor.

The output ripple is highest at maximum input voltage

since ∆IL increases with input voltage.

applicaTions inForMaTion

Setting Output Voltage

The LTC3895 output voltage is set by an external feed-

back resistor divider carefully placed across the output,

as shown in Figure 4a. The regulated output voltage is

determined by:

VOUT = 0.8V 1+

⎛

⎝

⎜

⎞

⎠

⎟

To improve the frequency response, a feedforward ca-

pacitor, CFF, may be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

The LTC3895 also has the option to be programmed to

a fixed 5V or 3.3V output through control of the VPRG

pin. Figure 4b shows how the VFB pin is used to sense

the output voltage in fixed output mode. Tying VPRG to

INTVCCorGNDprogramsVOUTto5Vor3.3V,respectively.

Floating VPRG sets VOUT to adjustable output mode using

external resistors.

3895 F04a

LTC3895

VFB

CFF

VOUT

3895 F04b

LTC3895

VFB

VPRG

INTVCC/GND

COUT

VOUT

5V/3.3V

(4a) Setting Adjustable Output Voltage

(4b) Setting Output to Fixed 5V/3.3V Voltage

Figure 4. Setting Output Voltage

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