LTC3890-2 datasheet(21/40 Pages) LINER

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LTC3890-2

38902f

1/2 LTC3890-2

VFB

VOUT

CFF

38902 F05

Figure 5. Setting Output Voltage

APPLICATIONS INFORMATION

The benefit of the LTC3890-2 2-phase operation can be

calculated by using Equation 1 for the higher power control-

ler 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 requirement calculated above for the

worst-case controller 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 share a common CIN(s). Separating

the drains and CIN may produce undesirable voltage and

current resonances at VIN.

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

VIN pin and ground, placed close to the LTC3890-2, is

also suggested. A 10Ω resistor placed between CIN (C1)

and the VIN pin provides further isolation between the

two channels.

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 (ΔVOUT) is approximated by:

ΔV

OUT ≈ΔIL ESR +

8• f • COUT

⎛

⎝

⎜

⎞

⎠

⎟

where f is the operating frequency, COUT is the output

capacitance and ΔIL is the ripple current in the inductor.

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage.

Setting Output Voltage

The LTC3890-2 output voltages are each set by an exter-

nal feedback resistor divider carefully placed across the

output, as shown in Figure 5. 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.

Tracking and Soft-Start (TRACK/SS Pins)

The start-up of each VOUT is controlled by the voltage on

the respective TRACK/SS pin. When the voltage on the

TRACK/SS pin is less than the internal 0.8V reference, the

LTC3890-2 regulates the VFB pin voltage to the voltage on

the TRACK/SS pin instead of 0.8V. The TRACK/SS pin can

be used to program an external soft-start function or to

allow VOUT to track another supply during start-up.

Soft-start is enabled by simply connecting a capacitor

from the TRACK/SS pin to ground, as shown in Figure 6.

An internal 1μA current source charges the capacitor,

providing a linear ramping voltage at the TRACK/SS pin.

The LTC3890-2 will regulate the VFB pin (and hence VOUT)

according to the voltage on the TRACK/SS pin, allowing

VOUT to rise smoothly from 0V to its final regulated value.

The total soft-start time will be approximately:

tSS = CSS •

0.8V

1μA

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