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LT8390A

8390afa

For more information www.linear.com/LT8390A

OPERATION

Main Control Loop

The LT8390A is a fixed frequency current mode control-

ler. The inductor current is sensed through the inductor

sense resistor between the LSP and LSN pins. The current

sense voltage is gained up by amplifier A1 and added to

a slope compensation ramp signal from the internal os-

cillator. The summing signal is then fed into the positive

terminals of the buck current comparator A3 and boost

current comparator A4. The negative terminals of A3 and

A4 are controlled by the voltage on the VC pin, which is

the diode-OR of error amplifiers EA1 and EA2.

Depending on the state of the peak-buck peak-boost cur-

rent mode control, either the buck logic or the boost logic

is controlling the four power switches so that either the

FB voltage is regulated to 1V or the current sense voltage

between the ISP and ISN pins is regulated by the CTRL

pin during normal operation. The gains of EA1 and EA2

have been balanced to ensure smooth transition between

constant-voltage and constant-current operation with the

same compensation network.

Light Load Current Operation

At light load, the LT8390A runs either at full switching fre-

quencydiscontinuousconductionmodeorpulse-skipping

mode, where the switches are held off for multiple cycles

(i.e., skipping pulses) to maintain the regulation and

improve the efficiency. Both the buck and boost reverse

current sense thresholds are set to 1mV (typical) so that

no reverse inductor current is allowed. Such no reverse

inductor current from the output to the input is highly

desired in certain applications.

In the buck region, switch B is turned off whenever the

buck reverse current threshold is triggered during (B+D)

phase. In the boost region, switch D is turned off whenever

the boost reverse current threshold is triggered during

(A+D) phase. In the buck-boost region, switch D is turned

off whenever the boost reverse current threshold is trig-

gered during (A+D) phase, and both switches B and D are

turned off whenever the buck reverse current threshold is

triggered during (B+D) phase.

Internal Charge Path

Each of the two top MOSFET drivers is biased from its

floating bootstrap capacitor, which is normally re-charged

by INTVCC through both the external and internal boot-

strap diodes when the top MOSFET is turned off. When

the LT8390A operates exclusively in the buck or boost

regions, one of the top MOSFETs is constantly on. An

internal charge path, from VOUT and BST2 to BST1 or from

VIN and BST1 to BST2, charges the bootstrap capacitor to

4.6V so that the top MOSFET can be kept on.

Shutdown and Power-On-Reset

The LT8390A enters shutdown mode and drains less than

2µA quiescent current when the EN/UVLO pin is below its

shutdown threshold (0.3V minimum). Once the EN/UVLO

pin is above its shutdown threshold (1V maximum), the

LT8390A wakes up startup circuitry, generates bandgap

reference, and powers up the internal INTVCC LDO. The

INTVCCLDOsuppliestheinternalcontrolcircuitryandgate

drivers. Now the LT8390A enters undervoltage lockout

(UVLO) mode with a hysteresis current (2.5µA typical)

pulled into the EN/UVLO pin. When the INTVCC pin is

charged above its rising UVLO threshold (3.78V typi-

cal), the EN/UVLO pin passes its rising enable threshold

(1.233V typical), and the junction temperature is less than

its thermal shutdown (165°C typical), the LT8390A enters

enable mode, in which the EN/UVLO hysteresis current is

turned off and the voltage reference VREF is being charged

up from ground. From the time of entering enable mode to

the time of VREF passing its rising UVLO threshold (1.89V

typical), the LT8390A is going through a power-on-reset

(POR), waking up the entire internal control circuitry and

settling to the right initial conditions. After the POR, the

LT8390A is ready and waiting for the signals on the CTRL

and LOADEN pins to start switching.

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