LTC3829 datasheet(14/40 Pages) LINER

14 / 40 page

LTC3829

3829f

operaTion (Refer to Functional Diagram)

A phase-locked loop (PLL) is available on the LTC3829

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 LTC3829. 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. The controller is operating in forced continuous

mode when it is synchronized.

Sensing the Output Voltage with a

Differential Amplifier

The LTC3829 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.

TheLTC3829differentialamplifierhasatypicaloutputslew

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

amplifierisINTVCC–1.4V(typically3.6V).Abovethisoutput

voltage the differential amplifier should not be used.

Power Good (PGOOD Pin)

The PGOOD pin is connected to an open drain of an

internal N-channel MOSFET. The MOSFET turns on and

pulls the PGOOD pin low when the 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 LTC3829 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 the VFBpiniswithintheregulationwindow.However,

there is an internal 100µs power-bad mask when the VFB

goes out of the 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 LTC3829 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 3.3V.

To prevent oscillation when there is a disturbance on the

INTVCC, the UVLO comparator has 600mV of precision

hysteresis.

Anotherwaytodetectanundervoltageconditionistomoni-

tor 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 comparator by adjusting the val-

ues of the resistive divider. For accurate VIN undervoltage

detection, VIN needs to be higher than 4.5V.

LTC3829 Datasheet(PDF) 14 Page - Linear Technology

LTC3829 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL