LTC3883 datasheet(12/22 Pages) LINER

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LTC3870

3870fb

For more information www.linear.com/LTC3870

TheTypicalApplicationonthefirstpageofthisdatasheetis

a basic LTC3870 application circuit featuring the LTC3880

as a slave controller. In paralleled operation, the current

sensing scheme as well as the power stage parameters

in LTC3870 must be the same as the master controller to

achieve balanced current sharing between masters and

slaves. Finally, input and output capacitors are selected

based on RMS current rating, ripple, and transient specs.

Current Limit Programming

To match the master controller current limit, each chan-

nel of LTC3870 can be programmed separately with two

current ranges. The ILIM pin of LTC3870 is a 4-level logic

input which sets the current limit of LTC3870. When ILIM

is grounded, both channel0 and channel1 are set to be low

current range. When ILIM is tied to INTVCC, both channel0

and channel1 are set to be high current range. Here, low

current range means the current sense threshold linearly

increases from 0mV to 50mV as ITH voltage is increased

from 0.5V to 2.22V without slope compensation. High cur-

rent range means the current sense threshold increases to

75mV as ITH voltage is increased to 2.22V without slope

compensation. Set ILIM to one-third INTVCC for channel0

high current range and channel1 low current range. Set

ILIM to two-thirds INTVCC or float for channel0 low current

range and channel1 high current range. The summary of

ILIM pin setups is shown in Table 2. For balanced load

current sharing, use the same current range setting as

in the master controller. Note that the LTC3870 does not

have active clamping circuit on ITH pin for peak current

limit and over current protection. Over current protection

relies on the master controller to drive and clamp the ITH

pinvoltagenottoexceedtheprogrammedvoltagethrough

the PMBus command.

Table 2.

ILIM

Channel 0

Current limit

Channel 1

Current limit

GND

Range Low

1/3 INTVCC

Range High

Range Low

2/3 INTVCC or Float

Range Low

Range High

INTVCC

Range High

applicaTions inForMaTion

INTVCC Regulators and EXTVCC

The LTC3870 features a PMOS LDO that supplies power

to INTVCC from the VIN supply. INTVCC powers the gate

drivers and most of the LTC3870’s internal circuitry. The

linear regulator regulates the voltage at the INTVCC pin

to 5.0V when VIN is greater than 6V. EXTVCC connects to

INTVCC through another PMOS LDO and can supply the

needed power when its voltage is higher than 4.8V and

VIN is higher than 6.5V. Each of these LDOs can supply a

peak current of 100mA and must be bypassed to ground

with a minimum of 4.7µF ceramic capacitor or low ESR

electrolytic capacitor. No matter what type of bulk capaci-

tor is used, an additional 0.1µF ceramic capacitor placed

directly adjacent to the INTVCC and PGND pins is highly

recommended. Good bypassing is needed to supply the

high transient currents required by the MOSFET gate

drivers and to prevent interaction between the channels.

HighinputvoltageapplicationsinwhichlargeMOSFETsare

being driven at high frequencies may cause the maximum

junctiontemperatureratingfortheLTC3870tobeexceeded.

TheINTVCCcurrent,whichisdominatedbythegatecharge

current, may be supplied by either the 5.0V linear regula-

tor from VIN or the linear regulator from EXTVCC. When

the voltage on the EXTVCC pin is less than 4.8V, the linear

regulator from VIN is enabled. Power dissipation for the

IC in this case is highest and is equal to VIN • IINTVCC. The

gate charge current is dependent on operating frequency.

The junction temperature can be estimated by using the

equations given in Note 2 of the Electrical Characteristics.

For example, the LTC3870 INTVCCcurrentislimitedtoless

than 34mA from a 38V supply in the UFD package and not

using the EXTVCC supply:

TJ = 70°C + (34mA)(38V)(43°C/W) = 125°C

whereambienttemperatureis70°Candthermalresistance

from junction to ambient is 43°C/W.

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current must be checked

while operating in continuous conduction mode (MODE

= INTVCC) at maximum VIN. When the voltage applied to

EXTVCC rises above 4.8V and VIN above 6.5V, the INTVCC

linearregulatoristurnedoffandtheEXTVCClinearregulator

is turned on. Using the EXTVCC allows the MOSFET driver

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