LTC4449 datasheet(28/48 Pages) AD

28 / 48 page

LTC7871

Rev. 0

For more information www.analog.com

If the external clock frequency is greater than the inter-

nal oscillator’s frequency, fOSC, then current is sourced

continuously from the phase detector output, pulling up

the filter network. When the external clock frequency is

less than fOSC, current is sunk continuously, pulling down

the filter network. If the external and internal frequencies

are the same but exhibit a phase difference, the current

sources turn on for an amount of time corresponding to

the phase difference. The voltage on the filter network is

adjusted until the phase and frequency of the internal and

external oscillators are identical. At the stable operating

point, the phase detector output is high impedance and

the filter capacitor holds the voltage.

Typically, the external clock (on the SYNC pin) input high

threshold is 2V, while the input low threshold is 1.2V. The

LTC7871 switching frequency is determined by:

Frequency = VFREQ • 414kHz/V – 163.5kHz

where, VFREQ = IFREQ (from spec table) • RFREQ

Or,

Frequency = RFREQ • 8.28kHz/kΩ– 163.5kHz

This assumes a perfect 20μA IFREQ.

Shared Pin Connections in Multichip Applications

When multiple LTC7871 ICs are used together in high cur-

rent applications, the customer may or may not connect

certain pins together, balancing better communication

between the ICs versus avoiding a single point failure.

The CLKOUT pin allows multiple LTC7871s to be daisy

chained together. The clock output signal on the CLKOUT

pin can be used to synchronize additional ICs in a multi-

phase power supply solution feeding a single high current

output, or even several outputs from the same input supply.

The SS and PWMEN pins should be tied together to enable

every LTC7871 IC to start up together. Not connecting

them together may result in some phases sourcing a lot

of current and others sinking current.

The IMON pins may or may not be tied together, depend-

ing on whether the customer wants to monitor the

average current per IC or the total average current in

the application.

flowing out of the FREQ pin. This allows the user to use

a single resistor to SGND to set the switching frequency

when no external clock is applied to the SYNC pin. The

internal switch between the FREQ pin and the integrated

PLL filter network is on, allowing the filter network to be

pre-charged at the same voltage as of the FREQ pin. The

relationship between the voltage on the FREQ pin and

operating frequency is shown in Figure 7 and specified in

the Electrical Characteristics table. If an external clock is

detected on the SYNC pin, the internal switch mentioned

above turns off and isolates the influence of the FREQ

pin. Note that the LTC7871 can only be synchronized to

an external clock whose frequency is within range of the

LTC7871’s internal VCO. A simplified block diagram is

shown in Figure 8.

Figure 7.

VFREQ (V)

0.5

1.5

2.5

200

400

600

800

1000

7871 F07

Relationship Between Oscillator Frequency

and Voltage at the FREQ Pin

Figure 8.

DIGITAL

PHASE/

FREQUENCY

DETECTOR

VCO

2.4V 5V

20µA

RFREQ

7871 F08

FREQ

SYNC

EXTERNAL

OSCILLATOR

SYNC

DIGITAL

PHASE/

FREQUENCY

DETECTOR

VCO

2.4V 5V

20µA

RFREQ

7871 F08

FREQ

SYNC

EXTERNAL

OSCILLATOR

SYNC

Phase-Locked Loop Block Diagram

APPLICATIONS INFORMATION

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