LTC1875 Просмотр технического описания (PDF) - Linear Technology

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LTC1875

15µA Quiescent Current 1.5A Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC1875

U

OPERATIO (Refer to Block Diagram)

Main Control Loop

The LTC1875 uses a constant frequency, current mode

step-down architecture. Both the top MOSFET and syn-

chronous bottom MOSFET switches are internal. During

normal operation, the internal top power MOSFET is

turned on each cycle when the oscillator sets the RS latch,

and turned off when the current comparator, ICOMP, resets

the RS latch. The peak inductor current at which ICOMP

turns the top MOSFET off is controlled by the voltage on

the ITH pin, which is the output of error amplifier EA. When

the load current increases, it causes a slight decrease in

the feedback voltage, VFB, relative to the 0.8V internal

reference, which, in turn, causes the ITH voltage to in-

crease until the average inductor current matches the new

load current. While the top MOSFET is off, the bottom

MOSFET is turned on until either the inductor current

starts to reverse direction or the next clock cycle begins.

Comparator OVDET guards against transient overshoots

> 7.5% by turning the main switch off and keeping it off

until the fault is removed.

Burst Mode Operation

The LTC1875 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand. To enable Burst Mode operation, simply

tie the SYNC/MODE pin to SVIN or connect it to a logic high

(VSYNC/MODE > 1.5V). To disable Burst Mode operation

and enable PWM pulse skipping mode, connect the SYNC/

MODE pin to SGND. In this mode, the efficiency is lower at

light loads but becomes comparable to Burst Mode opera-

tion when the output load exceeds 100mA. The advantage

of pulse skipping mode is lower output ripple.

When the converter is in Burst Mode operation, the peak

current of the inductor is set to approximately 400mA,

even though the voltage at the ITH pin indicates a lower

value. The voltage at the ITH pin drops when the inductor’s

average current is greater than the load requirement. As

the ITH voltage drops below approximately 0.45V, the

BURST comparator trips, turning off both power MOSFETs.

The ITH pin is then disconnected from the output of the EA

amplifier and held 0.65V above ground.

In sleep mode, both power MOSFETs are held off and the

internal circuitry is partially turned off, reducing the quies-

cent current to 15µA. The load current is now being

supplied from the output capacitor. When the output

voltage drops, the ITH pin reconnects to the output of the

EA amplifier and the top MOSFET is again turned on and

this process repeats.

Soft-Start/Run Function

The RUN/SS pin provides a soft-start function and a

means to shut down the LTC1875. Soft-start reduces the

input current surge by gradually increasing the regulator’s

maximum output current. This pin can also be used for

power supply sequencing.

Pulling the RUN/SS pin below 0.7V shuts down the

LTC1875, which then draws < 1µA current from the sup-

ply. This pin can be driven directly from logic circuits as

shown in Figure 1. It is recommended that this pin is driven

to VIN during normal operation. Note that there is no

current flowing out of this pin. Soft-start action is accom-

plished by connecting an external RC network to the RUN/

SS pin as shown in Figure 1. The LTC1875 actively pulls

the RUN/SS pin to ground under low input supply voltage

conditions.

VIN

3.3V OR 5V D1*

0.32V

RSS

*ZETEX BAT54

RUN/SS

CSS

1875 F01

Figure 1. RUN/SS Pin Interfacing

1875f

8

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