LTC3891 datasheet(21/38 Pages) LINER

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LTC3892/

LTC3892-1/LTC3892-2

38921fc

For more information www.linear.com/LTC3892

APPLICATIONS INFORMATION

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 LTC3892/LTC3892-1/

LTC3892-2, ceramic capacitors can also be used for CIN.

Always consult the manufacturer if there is any question.

The benefit of the LTC3892/LTC3892-1/LTC3892-2

2-phase operation can be calculated by using Equation 1

for the higher power controller and then calculating the

loss that would have resulted if both controller channels

switched on at the same time. The total RMS power lost

is lower when both controllers are operating due to the

reduced overlap of current pulses required through the

input capacitor’s ESR. This is why the input capacitor’s

requirementcalculatedabovefortheworst-casecontroller

is adequate for the dual controller design. Also, the input

protection fuse resistance, battery resistance, and PC

board trace resistance losses are also reduced due to the

reduced peak currents in a 2-phase system. The overall

benefit of a multiphase design will only be fully realized

when the source impedance of the power supply/battery

is included in the efficiency testing. The drains of the top

MOSFETs should be placed within 1cm of each other and

shareacommonCIN(s).SeparatingthedrainsandCINmay

produceundesirablevoltageandcurrentresonancesatVIN.

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

VINpinandground,placedclosetotheLTC3892/LTC3892-

1/LTC3892-2, is also suggested. A 2.2Ω to 10Ω resistor

placed between CIN (C1) and the VIN pin provides further

isolation, but is not required.

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:

∆VOUT ≈ ∆IL 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.

Setting Output Voltage

The LTC3892/LTC3892-1/LTC3892-2 output voltages are

setbyanexternalfeedbackresistordividercarefullyplaced

across the output, as shown in Figure 3a. 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.

For the LTC3892 and LTC3892-2, channel 1 has the

option to be programmed to a fixed 5V or 3.3V output

through control of the VPRG1 pin (not available on the

LTC3892-1). Figure 3b shows how the VFB1 pin is used

to sense the output voltage in fixed output mode. Tying

VPRG1 to INTVCC or GND programs VOUT1 to 5V or 3.3V,

respectively. Floating VPRG1 sets VOUT1 to adjustable

output mode using external resistors.

38921 F05a

1/2 LTC3892/

LTC3892-1/

LTC3892-2

VFB

CFF

VOUT

38921 F05b

LTC3892/

LTC3892-2

VFB1

VPRG1

INTVCC/GND

COUT

VOUT1

5V/3.3V

Figure 3. Setting Buck Output Voltage

(3a) Setting Adjustable Output Voltage

(3b) Setting CH1 (LTC3892) to Fixed 5V/3.3V Voltage

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