LT3757 datasheet(17/38 Pages) AD

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LTC7840

Rev 0

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OPERATION

the RUN pin is below 1.14V. The PGOOD pins will flag

power good immediately when their feedback voltages

are within ±10% of the reference window. However, there

is an internal 135µs power bad mask when feedback volt-

ages go out of the ±10% window. The PGOOD pins are

allowed to be pulled up by external resistors to sources

of up to 6V.

Multichip Operations (CLKOUT Pin)

The LTC7840 uses CLKOUT pin to allow multiple ICs to

be daisy-chained together for higher current multi-phase

applications. For a 4-phase design, the CLKOUT signal

of the master controller is connected to the SYNC input

of the slave controller in order to synchronize additional

power stages for a single high current output. Input

capacitance ESR requirements and efficiency losses are

substantially reduced because the peak current drawn from

the input capacitor is effectively divided by the number of

phases used, and power loss is proportional to the RMS

current squared. A two stage, single output voltage imple-

mentation can reduce input path power loss by 75% and

radically reduce the required RMS current rating of the input

capacitor(s).

In LTC7840, Channel 1 and Channel 2 have 180º phase

shift, whereas, CLKOUT signal has 90º phase shift relative

to Channel 1. The duty cycle of CLKOUT signal is 50%.

Using the LTC7840 Transconductance (gm) Error

Amplifier in Multi-Phase Applications

The LTC7840 error amplifier is a transconductance, or gm

amplifier, meaning that it has high DC gain but high output

impedance (the output of the error amplifier is a current

proportional to the differential input voltage). This style

of error amplifier greatly eases the task of implement-

ing a multi-phase solution, because the amplifiers from

two or more chips can be connected in parallel. In this

case the FB pins of multiple LTC7840s can be connected

together, as well as the ITH pins, as shown in Figure 7. The

gm of the composite error amplifier is simply n times the

transconductance of one amplifier, or gm(TOT) = n • 800μS,

where n is the number of amplifiers connected in paral-

lel. The transfer function from the ITH pin to the current

comparator inputs was carefully designed to be accurate,

both from channel-to-channel and chip-to-chip. This way

the peak inductor current matching is kept accurate.

Figure 7.

FREQ

VFB1

VFB2

CLKOUT

SYNC

LTC7840

MASTER

SGND

ITH

VOUT

DRVCC

SS1

SS2

RUN1

RUN2

ON/OFF

CONTROL

ALL RUN PINS

CONNNECTED

TOGETHER

INDIVIDUAL

DRVCC PINS

LOCALLY

DECOUPLED

FREQ

VFB1

VFB2

†

CLKOUT

SYNC

LTC7840

SLAVE

SGND

ITH

DRVCC

SS1

SS2

RUN1

RUN2

7840 F06

ALL SS PINS

CONNNECTED

TOGETHER

ALL FB PINS

CONNECTED

TOGETHER

ALL ITH PINS

CONNECTED

TOGETHER

* R = 100

† CX = 100pF

LTC7840 Error Amplifier Configuration for Multi-Phase Operation

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