LTC3853_15 datasheet(19/36 Pages) LINER

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LTC3853

3853fc

For more information www.linear.com/LTC3853

Figure 7. Equivalent Input Circuit of Error Amplifier

Figure 5. Two Different Modes of Output Voltage Tracking

Figure 6. Setup for Coincident and Ratiometric Tracking

APPLICATIONS INFORMATION

TIME

(5a) Coincident Tracking

VOUT1

VOUT2

3853 F05a

VOUT1

VOUT2

TIME

3853 F05b

(5b) Ratiometric Tracking

VOUT2

(6a) Coincident Tracking Setup

VFB1

TK/SS2

VFB2

VOUT1

VOUT2

3853 F06

(6b) Ratiometric Tracking Setup

VFB1

TK/SS2

VFB2

VOUT1

–

TK/SS2

0.8V

VFB2

3853 F07

EA2

coincident mode, the TK/SS voltage is substantially higher

than 0.8V at steady state and effectively turns off D1. D2

and D3 will therefore conduct the same current and offer

tight matching between VFB2 and the internal precision

0.8V reference. In the ratiometric mode, however, TK/SS

equals 0.8V at steady state. D1 will divert part of the bias

current to make VFB2 slightly lower than 0.8V.

Although this error is minimized by the exponential I-V

characteristic of the diode, it does impose a finite amount

of output voltage deviation. Furthermore, when the master

channel’s output experiences dynamic excursion (under

load transient, for example), the slave channel output will

be affected as well. For better output regulation, use the

coincident tracking mode instead of ratiometric.

INTVCC Regulators and EXTVCC

TheLTC3853featuresanNPNlinearregulatorthatsupplies

power to INTVCC from the VIN supply. INTVCC powers the

gate drivers and much of the LTC3853’s internal circuitry.

The linear regulator regulates the voltage at the INTVCCpin

to 5V when VIN is greater than 6.5V. EXTVCC connects to

INTVCC through a P-channel MOSFET and can supply the

needed power when its voltage is higher than 4.7V. Each

of these can supply a peak current of 150mA and must

be bypassed to ground with a minimum of 1µF ceramic

capacitor or low ESR electrolytic capacitor. No matter

what type of bulk capacitor is used, an additional 0.1µF

ceramic capacitor placed directly adjacent to the INTVCC

and PGND pins is highly recommended. Good bypassing

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