LTC1629I データシートの表示（PDF） - Linear Technology

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LTC1629I

PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators

Linear Technology 

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OPERATIO (Refer to Functional Diagram)

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

of the IC circuitry is derived from INTVCC. When the

EXTVCC pin is left open, an internal 5V low dropout

regulator supplies INTVCC power. If the EXTVCC pin is

taken above 4.7V, the 5V regulator is turned off and an

internal switch is turned on connecting EXTVCC to INTVCC.

This allows the INTVCC power to be derived from a high

efficiency external source such as the output of the regu-

lator itself or a secondary winding, as described in the

Applications Information section. An external Schottky

diode can be used to minimize the voltage drop from

EXTVCC to INTVCC in applications requiring greater than

the specified INTVCC current. Voltages up to 7V can be

applied to EXTVCC for additional gate drive capability.

Differential Amplifier

This amplifier provides true differential output voltage

sensing. Sensing both VOUT+ and VOUT– benefits regula-

tion in high current applications and/or applications hav-

ing electrical interconnection losses. The AMPMD pin

(available on the LTC1629 only) allows selection of inter-

nal precision feedback resistors for high common mode

rejection differencing applications, or direct access to the

actual amplifier inputs without these internal feedback

resistors for other applications. The AMPMD pin is

grounded to connect the internal precision resistors in a

unity-gain differencing application or tied to the INTVCC

pin to bypass the internal resistors and make the amplifier

inputs directly available. The amplifier is a unity-gain

stable, 2MHz gain-bandwidth, >120dB open-loop gain

design. The amplifier has an output slew rate of 5V/µs and

is capable of driving capacitive loads with an output RMS

current typically up to 25mA. The amplifier is not capable

LTC1629/LTC1629-PG

of sinking current and therefore must be resistively loaded

to do so. The differential amplifier is configured as a unity-

gain differencing amplifier in the LTC1629-PG.

Power Good (PGOOD) (LTC1629-PG Only)

The PGOOD pin is connected to the drain of an internal

MOSFET. The MOSFET turns on when the output is not

within ±7.5% of its nominal output level as determined by

the feedback divider. When the output is within ±7.5% of

its nominal value, the MOSFET is turned off within 10µs

and the PGOOD pin should be pulled up by an external

resistor to a source of up to 7V.

Short-Circuit Detection

The RUN/SS capacitor is used initially to limit the inrush

current from the input power source. Once the controllers

have been given time, as determined by the capacitor on

the RUN/SS pin, to charge up the output capacitors and

provide full load current, the RUN/SS capacitor is then

used as a short-circuit timeout circuit. If the output voltage

falls to less than 70% of its nominal output voltage the

RUN/SS capacitor begins discharging assuming that the

output is in a severe overcurrent and/or short-circuit

condition. If the condition lasts for a long enough period

as determined by the size of the RUN/SS capacitor, the

controller will be shut down until the RUN/SS pin voltage

is recycled. This built-in latchoff can be overidden by

providing a >5µA pull-up current at a compliance of 5V to

the RUN/SS pin. This current shortens the soft-start

period but also prevents net discharge of the RUN/SS

capacitor during a severe overcurrent and/or short-circuit

condition. Foldback current limiting is activated when the

output voltage falls below 70% of its nominal level whether

or not the short-circuit latchoff circuit is enabled.

APPLICATIO S I FOR ATIO

The basic LTC1629 application circuit is shown in Figure␣ 1

on the first page. External component selection is driven

by the load requirement, and begins with the selection of

RSENSE1, 2. Once RSENSE1, 2 are known, L1 and L2 can be

chosen. Next, the power MOSFETs and D1 and D2 are

selected. The operating frequency and the inductor are

chosen based mainly on the amount of ripple current.

Finally, CIN is selected for its ability to handle the input

ripple current (that PolyPhase operation minimizes) and

COUT is chosen with low enough ESR to meet the output

ripple voltage and load step specifications (also minimized

with PolyPhase). The circuit shown in Figure␣ 1 can be

configured for operation up to an input voltage of 28V

(limited by the external MOSFETs).

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