LTC7899 datasheet(20/50 Pages) AD

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Data Sheet

LTC7899

analog.com

Rev. 0

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THEORY OF OPERATION

Main Control Loop

The LTC7899 is a synchronous controller utilizing a constant frequency, peak current mode architecture. During

normal operation, the external bottom MOSFET turns on when the clock sets the RS latch, causing the inductor

current to increase. The main switch turns off when the main current comparator, ICMP, resets the RS latch. After

the bottom MOSFET is turned off each cycle, the top MOSFET turns on, which causes the inductor current to decrease

until either the inductor current starts to reverse, as indicated by the current comparator (IR), or the beginning of the

next clock cycle.

The peak inductor current at which ICMP trips and resets the latch is controlled by the voltage on the ITH pin, which

is the output of the error amplifier (EA). The error amplifier compares the output voltage feedback signal at the VFBB

pin (which is generated with an external resistor connected to VIN–) to the internal 1.2V reference voltage. When the

load current increases, it causes a slight decrease in VFBB relative to the reference, which causes the EA to increase

the ITH voltage until the average inductor current meets the new load current.

After the bottom MOSFET is turned off each cycle, the top MOSFET turns on until either the inductor current starts to

reverse, as indicated by the current comparator (IR), or the beginning of the next clock cycle.

Power and Bias Supplies (V

BIAS, EXTVCC, DRVCC, and INTVCC)

Power for the top and bottom MOSFET drivers is derived from the DRVCC pin. LDOs (low dropout linear regulators)

are available from both the VBIAS and EXTVCC pins to provide power to DRVCC, which can be programmed from 5V to

10V through control of the DRVSET pin. When the EXTVCC pin is tied to a voltage below its switchover voltage, the VBIAS

LDO supplies power to DRVCC. If EXTVCC is taken above its switchover voltage (4.8V, 6.1V or 7.7V depending on the

DRVUV pin), the VBIAS LDO is turned off and the EXTVCC LDO is turned on. Once enabled, the EXTVCC LDO supplies power

to DRVCC. Using the EXTVCC pin allows the DRVCC power to be derived from a high efficiency external source.

The INTVCC supplies power for most of the internal circuits in the LTC7899. The INTVCC LDO regulates to a fixed value

of 4.5V and its power is derived from the DRVCC supply.

High-Side Bootstrap Capacitor

The top MOSFET driver is biased from the floating bootstrap capacitor (CB), which normally recharges during each

cycle through an external diode between BOOST and DRVCC whenever the switch voltage goes low. The LTC7899 also

has an internal charge pump that keeps the required bias on BOOST. The charge pump always operates in both

forced continuous mode and pulse-skipping mode. In burst mode operation, the charge pump is turned off during

sleep and enabled when the chip wakes up. The internal charge pump can nominally supply a charging current of

80µA.

If the input voltage GND to VIN– decreases to a voltage close to its output (VOUT), the loop may enter dropout and

attempt to turn on the top MOSFET continuously. In forced continuous mode or pulse-skipping mode, the internal

charge pump enables the top MOSFET to be turned on continuously, resulting in a 100% duty cycle.

Dead Time Control (DTCA and DTCB Pins)

The LTC7899 dead time delays can be programmed from 13ns to 100ns through configuration of the DTCA and DTCB

pins. The DTCA pin programs the dead time associated with the bottom MOSFET turning off (BG falling) and the top

MOSFET turning on (TG rising). The DTCB pin programs the dead time associated with the top MOSFET turning off

(TG falling) and the bottom MOSFET turning on (BG rising).

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