LTC3613 datasheet(16/44 Pages) AD

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LTC7131-1

Rev. 0

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OPERATION

slowly at a rate determined by the soft-start capacitor value.

The SS/VOUT ramp-down rate is calculated by Equation 1.

dV/dt = 5µA/CSS

(1)

When the output voltage has reached zero, the LTC7131-1

will turn the drivers off and shut down. Soft off is the

default power down mode.

Immediate off mode is selected if the SOFT OFF bit is

low when the LTC7131-1 is turned off. In this mode,

the LTC7131-1 turns its drivers off immediately and the

output voltage discharge rate is a function of the output

capacitor value and the load.

Short-Circuit Protection

The LTC7131-1 has a precision cycle-by-cycle current

limit to prevent inductor saturation in a short-circuit con-

dition. The valley of the inductor current is guaranteed to

not exceed IMAX ±10% where IMAX is set by the ILIM pin.

The maximum cycle-by-cycle inductor current is therefore

limited to IMAX + 10% + ΔIL, where ΔIL depends on the

inductor valley and operating frequency but is typically

40% of IMAX. Internal control circuitry also guarantees

smooth recovery with no output voltage overshoot once

the short is removed.

25MHz Error Amplifier and Remote Sense

Differential Amplifier

The LTC7131-1 utilizes a 25MHz error amplifier and dif-

ferential amplifier for fast and accurate output voltage

regulation. The operational amplifier style error amplifier

allows precision tuning of the system poles and zeros

for optimal transient response. The remote sense dif-

ferential amplifier allows output voltage sensing at the

point-of-load and thus provides very accurate regulation

of the output voltage and telemetry readback regardless

of load current. The sensed output voltage is available at

the DAOUT pin (referenced to SGND). This pin is typically

connected to the FB pin which is the error amplifier “-”

input. See Figure 1.

Using Separate SVIN/VIN Supplies

The LTC7131-1 has two supply pins: SVIN that supplies

the control circuitry and gate drivers and VIN that supplies

the power switches. SVIN requires a minimum of 4.7V,

while VIN may be able to supply power to the load at lower

voltages. To maximize the lower operating range of the

supply voltage, two separate supplies can be used – a

SVIN supply that is > 4.7V and rated at 50mA or higher

and a VIN supply, rated for the load, that can be run all

the way into dropout.

Thermal Warning and Thermal Shutdown

The LTC7131-1 has two levels of thermal thresholds and

two levels of responses. When the internal die temperature

exceeds 150°C, the overtemperature bit in the STATUS_

WORD is set and the ALERT pin pulls low to alert the PMBus

master. If the temperature continues to rise and exceeds

170°C, the LTC7131-1 shuts down all circuitry, including

output regulation, and will no longer respond to the PMBus

host. Both temperature monitors have about 20°C of hys-

teresis before the overtemperature condition is cleared. The

temperature warning bit in the STATUS_WORD is latched

and remains set until the host clears it.

2-Phase Operation

For output loads that demand more than 25A of current,

two LTC7131-1’s can be paralleled to run out-of-phase to

provide up to 50A output current. To configure a 2-phase

system, one LTC7131-1 will act as a master and the other

a slave (see the schematic in the Typical Applications

section). Connecting the VSENSE– pin to INTVCC puts the

LTC7131-1 in slave mode by tri-stating its error amplifier

and remote sense amplifier. The ITH pins of both IC’s are

connected together so that both are regulating the induc-

tor current based on the master’s ITH voltage. The mas-

ter’s CLKOUT pin is a clock waveform that is 180° out-of-

phase to its internal clock. This CLKOUT can be connected

to the MODE/SYNC pin of the slave to force the slave’s

PLL to lock onto this clock input and run out-of-phase

Figure 1. Differential Amplifier Connection

DAOUT

LTC7131-1

VSENSE+

COUT

VOUT

VSENSE–

7131-1 F01

–

DIFFAMP

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