LTC3636IUFD-1-PBF 查看數據表（PDF） - Linear Technology

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LTC3636IUFD-1-PBF

Dual Channel 6A, 20V Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC3636/LTC3636-1

Operation

The LTC3636/LTC3636-1 is a dual-channel, current mode

monolithic step down regulator capable of providing 6A

of output current from each channel. Its unique controlled

on-time architecture allows extremely low step-down

ratios while maintaining a constant switching frequency.

Both channels share the same clock and run 180° out of

phase. Each channel is enabled by raising the voltage on

the RUN pin above 1.22V nominally.

Main Control Loop

In normal operation, the internal top power MOSFET is

turned on for a fixed interval determined by a fixed one-

shot timer (“ON” signal in Block Diagram). When the top

power MOSFET turns off, the bottom power MOSFET turns

on until the current comparator ICMP trips, thus restarting

the one shot timer and initiating the next cycle. Inductor

current is measured by sensing the voltage drop across

the SW and GND nodes of the bottom power MOSFET. The

voltage on the ITH pin sets the comparator threshold cor-

responding to inductor valley current. The error amplifier

EA adjusts this ITH voltage by comparing an internal 0.6V

reference to the feedback signal VFB derived from the output

voltage. If the load current increases, it causes a drop in

the feedback voltage relative to the internal reference. The

ITH voltage then rises until the average inductor current

matches that of the load current.

The operating frequency is determined by the value of the

RT resistor, which programs the current for the internal

oscillator. An internal phase-locked loop servos the switch-

ing regulator on-time to track the internal oscillator edge

and force a constant switching frequency. A clock signal

can be applied to the MODE/SYNC pin to synchronize the

switching frequency to an external source. The regulator

defaults to forced continuous operation once the clock

signal is applied.

At light load currents, the inductor current can drop to zero

and become negative. In Burst Mode operation, a current

reversal comparator (IREV) detects the negative inductor

current and shuts off the bottom power MOSFET, result-

ing in discontinuous operation and increased efficiency.

Both power MOSFETs will remain off until the ITH voltage

rises above the zero current level to initiate another cycle.

During this time, the output capacitor supplies the load

current and the part is placed into a low current sleep

mode. Discontinuous mode operation is disabled by tying

the MODE/SYNC pin to ground, which forces continuous

synchronous operation regardless of output load current.

“Power Good” Status Output

The PGOOD open-drain output will be pulled low if the

regulator output exits a ±8% window around the regulation

point. This condition is released once regulation within a

±5% window is achieved. To prevent unwanted PGOOD

glitches during transients or dynamic VOUT changes, the

LTC3636/LTC3636-1 PGOOD falling edge includes a filter

time of approximately 40µs.

VIN Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient input voltage spikes, the LTC3636/

LTC3636-1 constantly monitors each VIN pin for an

overvoltage condition. When VIN rises above 22.5V, the

regulator suspends operation by shutting off both power

MOSFETs on the corresponding channel. Once VIN drops

below 21.5V, the regulator immediately resumes normal

operation. The regulator executes its soft-start function

when exiting an overvoltage condition.

Overcurrent and Short-Circuit Protection

The LTC3636 protects itself against output overcurrent

and short-circuits by sensing the inductor valley current.

When the current limit is reached, the output begins to fall,

resulting in decreased on-time of the top power MOSFET.

If the short is prolonged enough for the on-time to reach

its minimum, the off-time will lengthen, lowering the

switching frequency and preventing excess current from

being drawn from VIN. After the overcurrent or short is

removed, the regulator executes its soft-start function to

prevent the output voltage from overshooting.

For more information www.linear.com/LTC3636

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