LTC1705 データシートの表示（PDF） - Linear Technology

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LTC1705

Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO

Linear Technology 

LTC1705

APPLICATIO S I FOR ATIO

The RUN/SS pin shuts down the LTC1705 if it falls below

0.5V (Figure 4). Between 0.5V and about 1V, the LTC1705

wakes up and the duty cycle is kept to a miminum. As the

potential at RUN/SS increases, the duty cycle increases

linearly between 1V and 2V, reaching its final value of 90%

when RUN/SS is above 2V. Somewhere before this point,

the feedback amplifier will assume control of the loop and

the output will come into regulation. When RUN/SS rises

to 1V below VCC , the MIN feedback comparator is enabled

and the LTC1705 is in full operation.

VOUT

0V

START UP

5V

NORMAL

OPERATION

CURRENT

LIMIT

4V

COMP CONTROLS MIN

DUTY CYCLE COMPARATOR

ENABLE

2V

RUN/SS CONTROLS

DUTY CYCLE

1V

MINIMUM DUTY CYCLE

0.5V

0V

POWER-DOWN MODE

LTC1705 ENABLE

1705 F04

Figure 4. Soft-Start Operation in Start Up and Current Limit

Current Limit

The LTC1705 includes an onboard current limit circuit that

limits the maximum output current to a user-programmed

level. It works by sensing the voltage drop across QB

during the time that QB is on and comparing that voltage

to a user-programmed voltage at IMAX. Since QB looks like

a low value resistor during its on-time, the voltage drop

across it is proportional to the current flowing in it. In a

buck converter, the average current in the inductor is equal

to the output current. This current also flows through QB

during its on-time. Thus, by watching the voltage across

QB, the LTC1705 can monitor the output current.

Any time QB is on and the current flowing to the output is

reasonably large, the SW node at the drain of QB will be

somewhat negative with respect to PGND. The LTC1705

senses this voltage, inverts it and compares the sensed

voltage with a positive voltage at the IMAX pin. The IMAX pin

includes a trimmed 10µA pull-up, enabling the user to set

the voltage at IMAX with a single resistor, RIMAX, to ground.

The LTC1705 compares the two inputs and begins limiting

the output current when the magnitude of the negative

voltage at the SW pin is greater than the voltage at IMAX .

When the load current increases abruptly, the voltage

feedback loop forces the duty cycle to increase rapidly and

the on-time of QB will be small momentarily. The RDS(ON)

of QB must be low enough to ensure that the SW node is

pulled low within the QB on-time for proper current

sensing.

The current limit detector is connected to an internal gm

amplifier that pulls a current from the RUN/SS pin propor-

tional to the difference in voltage magnitudes between the

SW and IMAX pins. The maximum value of this current is

250µA (typically). It begins to discharge the soft-start

capacitor at RUN/SS, reducing the duty cycle and control-

ling the output voltage until the current drops below the

limit. The soft-start capacitor needs to move a fair amount

before it has any effect on the duty cycle, adding a delay

until the current limit takes effect (Figure 4). This allows

the LTC1705 to experience brief overload conditions with-

out affecting the output voltage regulation. The delay also

acts as a pole in the current limit loop to enhance loop

stability. While the soft-start capacitor is being discharged,

the top MOSFET must withstand the high power dissipa-

tion due to the high current especially if the regulator is

powered by a high current supply. Larger overloads cause

the soft-start capacitor to pull down quickly, protecting the

output components from damage. The current limit gm

amplifier includes a clamp to prevent it from pulling RUN/

SS below 0.5V and shutting off the LTC1705.

Power MOSFET RDS(ON) varies from MOSFET to MOSFET,

limiting the accuracy obtainable from the LTC1705 current

limit loop. Additionally, ringing on the SW node due to

parasitics can add to the apparent current, causing the

loop to engage early. The LTC1705 current limit is de-

signed primarily as a disaster prevention, “no blow up”

circuit and is not useful as a precision current regulator. It

should typically be set around 50% above the maximum

expected normal output current to prevent component

tolerances from encroaching on the normal current range.

See the Current Limit Programming section for advice on

choosing a valve for RIMAX .

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