LTC3613 datasheet(21/44 Pages) AD

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LTC7131-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The basic LTC7131-1 application circuit is shown in

Figure 10.

Current Limit Programming

The maximum current limit for the controller can be set to

any value in the range of 10A to 25A by the ILIM pin. For

a 25A limit, tie ILIM to INTVCC. For a 15A limit, tie ILIM

to SGND. For other limits, use Equation 2 to choose the

required ILIM voltage

Valley Current Limit (A) = 20 • VILIM + 1

(2)

Then use a resistive divider connected to INTVCC to set

ILIM to the chosen voltage.

Note that this current limit corresponds to the valley of

the inductor ripple so the maximum load current will be

higher by ΔI/2.

Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low

output ripple voltage. The operating frequency of the

LTC7131-1 is determined by an external resistor that is

connected between the RT pin and ground. The value of

the resistor sets the ramp current that is used to charge

and discharge an internal timing capacitor within the

oscillator and can be calculated by Equation 3.

RT(Ω) =

1.235 • 1010

fOSC

− 950

(3)

Frequencies as high as 2MHz are possible, as long as the

minimum on-time requirement is met (see next section).

Tying the RT pin to INTVCC sets the default internal oper-

ating frequency to 480kHz ±15%.

The LTC7131-1’s internal oscillator can be synchronized

to an external frequency by applying a square wave clock

signal to the MODE/SYNC pin. During synchronization,

the top switch turn-on is locked to the rising edge of the

external frequency source. The synchronization frequency

range is 250kHz to 2MHz. During synchronization, dis-

continuous operation is disabled.

The internal PLL has a synchronization range of ±30%

around its programmed frequency. Therefore, during

external clock synchronization be sure that the external

clock frequency is within this ±30% range of the RT pro-

grammed frequency.

When using the RT pin to program the oscillator fre-

quency, a square wave clock that is running 180° out-

of-phase with the internal oscillator is available at the

CLKOUT pin for connection to a second LTC7131-1 for

2-phase operation (see the 2-Phase section).

Figure 10. 25A Step-Down Regulator

INTVCC

LTC7131-1

BOOST

1µF

VOUT

0.4V TO 3.5V

25A

VIN

5V TO 20V

7131-1 F10

VIN SVIN

SGND

PGND

VSENSE+

VSENSE−

DAOUT

PGFD

ASEL

PMBus

INTERFACE

CSLEW

ILIM

MODE/SYNC

MARGIN

ITH

RC2

RREF2

RREF1

RC1

CC2

CIN

100µF

COUT

330µF

×2

CSS

REF

PGLIM

TRACK/SS

0.1µF

RUN_STBY

RUN_MSTR

SCL, SDA, ALERT

PGOOD

CLKOUT

CC1

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