LTC3856 datasheet(14/40 Pages) LINER

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LTC3856

3856f

operaTion (Refer to Functional Diagram)

operating frequency from 250kHz to 770kHz. There is a

precision10µAcurrentflowingoutoftheFREQpinenabling

the user to program the controller’s switching frequency

with a single resistor to SGND. A curve is provided later in

theApplicationsInformationsectionshowingtherelation-

ship between the voltage on the FREQ pin and switching

frequency.

A phase-locked loop (PLL) is available on the LTC3856

to synchronize the internal oscillator to an external clock

source that is connected to the PLLIN pin. The PLL loop

filter network is integrated inside the LTC3856. The

phase-locked loop is capable of locking any frequency

within the range of 250kHz to 770kHz. The frequency

setting resistor should always be present to set the

controller’s initial switching frequency before locking to

the external clock.

Sensing the Output Voltage with a Differential Amplifier

The LTC3856 includes a low offset, unity-gain, high band-

widthdifferentialamplifierforapplicationsthatrequiretrue

remote sensing. Sensing the load across the load capaci-

torsdirectlygreatlybenefitsregulationinhighcurrent,low

voltage applications, where board interconnection losses

can be a significant portion of the total error budget.

TheLTC3856differentialamplifierhasatypicaloutputslew

rate of 2V/µs. The amplifier is configured for unity gain,

meaning that the difference between DIFFP and DIFFN is

translated to DIFFOUT, relative to SGND.

Care should be taken to route the DIFFP and DIFFN PCB

traces parallel to each other all the way to the terminals

of the output capacitor or remote sensing points on the

board. In addition, avoid routing these sensitive traces

near any high speed switching nodes in the circuit. Ideally,

the DIFFP and DIFFN traces should be shielded by a low

impedance ground plane to maintain signal integrity.

The maximum output voltage when using the differential

amplifier is INTVCC – 1.4V (typically 3.6V). The differential

amplifier should not be used above this voltage.

Power Good (PGOOD Pin)

ThePGOODpinisconnectedtoanopendrainofaninternal

N-channel MOSFET. The MOSFET turns on and pulls the

PGOOD pin low when either VFB pin voltage is not within

±10% of the 0.6V reference voltage. The PGOOD pin is

also pulled low when the RUN pin is below 1.22V or when

the LTC3856 is in the soft-start or tracking phase. When

the VFB pin voltage is within the ±10% regulation window,

the MOSFET is turned off and the pin is allowed to be

pulled up by an external resistor to a source of up to 6V.

The PGOOD pin will flag power good immediately when

VFB is within the regulation window. However, there is an

internal 20µs power-bad mask when VFB goes out of the

regulation window.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>10%) as well as other more serious condi-

tions that may overvoltage the output. In such cases, the

topMOSFETisturnedoffandthebottomMOSFETisturned

on until the overvoltage condition is cleared.

Undervoltage Lockout

The LTC3856 has two functions that help protect the

controller in case of undervoltage conditions. A precision

UVLO comparator constantly monitors the INTVCCvoltage

to ensure that an adequate gate-drive voltage is present. It

locks out the switching action when INTVCC is below 3.2V.

To prevent oscillation when there is a disturbance on the

INTVCC, the UVLO comparator has 600mV of precision

hysteresis.

Another way to detect an undervoltage condition is

to monitor the VIN supply. Because the RUN pin has

a precision turn-on reference of 1.22V, one can use a

resistor divider to VIN to turn on the IC when VIN is high

enough. An extra 4.5µA of current flows out of the RUN

pin once the RUN pin voltage passes 1.22V. The RUN

comparator itself has about 80mV of hysteresis. One

can program additional hysteresis for the RUN com-

parator by adjusting the values of the resistive divider.

For accurate VIN undervoltage detection, VIN needs to

be higher than 4.5V.

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