Data Sheet

LT7200S

analog.com

Rev 0

15 of 35

THEORY OF OPERATION

Main Control Loop

The LT7200S is a quad-channel, current-mode monolithic step-down regulator capable of providing ±5A of output

current from each channel. In normal operation, the internal top power MOSFET is turned on for a fixed interval

determined by a 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, restarting the one-shot timer and initiating the next

cycle. Inductor current is determined by sensing the voltage drop across SW and GND nodes of the bottom power

MOSFET when it is on. The voltage on the ITH pin sets the comparator threshold corresponding to the inductor valley

current. The error amplifier, EA, adjusts this ITH voltage by comparing the feedback signal, VFB, with an internal 0.5V

reference. 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.

At light load currents, the inductor current can drop to zero and become negative. In Discontinuous Mode (DCM)

operation, this is detected by the current reversal comparator, IREV, which then shuts off the bottom power MOSFET.

Both power MOSFETs remain off with the output capacitor supplying the load current until the ITH voltage rises

above zero current level to initiate the next cycle. If continuous mode of operation is desired, simply float the

MODE/SYNC pin or tie it to INTVCC.

The operating frequency is determined by the value of the RRT resistor, which programs the current for the internal

oscillator. An internal phase-lock loop servos the switching regulator on-time to track the internal oscillator 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 clock. The regulator defaults to force continuous operation once the clock signal is present.

The “S” in LTC7200S refers to the second generation Silent Switcher technology. The IC has integrated ceramic

capacitors for VIN, INTVCC, and BOOST to keep all the fast AC current loops small, thus improving the EMI

performance. Furthermore, it allows for faster switching edges, which greatly improves efficiency at high switching

frequencies.

“Power Good” Status Output

PGOOD open-drain output is pulled low if the regulator output feedback voltage, VFB, exits a ±7.5% window around

the regulation point while the overvoltage (OV) or undervoltage (UV) comparator is tripped. This condition is released

once regulation within a ±5.5% window is achieved.

Continuous operation is forced during OV and UV conditions, except during start-up when the TRACK pin is ramping

up to 0.5V.

V

IN Overvoltage Protection

To protect the internal power MOSFET devices against transient voltage spikes, the LT7200S constantly monitors the

PVIN pin for an overvoltage condition. When the PVIN rises above 20V, the regulator suspends operation by shutting

off both power MOSFETs. Once PVIN drops below 18V, the regulator immediately resumes normal operation. During

an overvoltage event, the internal soft-start voltage is clamped to a voltage slightly higher than the feedback voltage.

Thus, the soft-start feature is present upon exiting an overvoltage condition.

Overcurrent and Short-Circuit Protection

The LT7200S 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, decreasing on-time of the top power MOSFET. If the short

is prolonged enough for the on-time to reach its minimum, the off-time lengthens, lowering the switching frequency