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LT8390

8390fa

For more information www.linear.com/LT8390

APPLICATIONS INFORMATION

MakesuretheCSSisatleastfivetotentimeslargerthanthe

compensation capacitor on the VCpinforawell-controlled

output voltage soft-start. A 0.1µF ceramic capacitor is a

good starting point.

The SS pin is also used as a fault timer. Once an output

short-circuit fault is detected, a 1.25µA pull-down current

source is activated. Using a single resistor from the SS

pin to the VREF pin, the LT8390 can be set to three differ-

ent fault protection modes: hiccup (no resistor), latch-off

(499kΩ), and keep-running (100kΩ).

With a 100kΩ resistor in keep-running mode, the LT8390

continues switching normally and regulates the current

into ground. With a 499kΩ resistor in latch-off mode, the

LT8390 stops switching until the EN/UVLO pin is pulled

low and high to restart. With no resistor in hiccup mode,

the LT8390 enters low duty cycle auto-retry operation. The

1.25µA pull-down current discharges the SS pin to 0.2V

and then 12.5µA pull-up current charges the SS pin up. If

the output short-circuit condition has not been removed

when the SS pin reaches 1.75V, the 1.25µA pull-down

current turns on again, initiating a new hiccup cycle. This

will continue until the fault is removed. Once the output

short-circuit condition is removed, the output will have a

smooth short-circuit recovery due to soft-start.

Loop Compensation

TheLT8390usesaninternaltransconductanceerrorampli-

fier,theoutputofwhich,VC,compensatesthecontrolloop.

Theexternalinductor,outputcapacitor,andthecompensa-

tion resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor on the VC pin are set to optimize control

loop response and stability. For a typical voltage regulator

application, a 10nF compensation capacitor on the VC pin

is adequate, and a series resistor should always be used

to increase the slew rate on the VC pin to maintain tighter

output voltage regulation during fast transients on the

input supply of the converter.

Efficiency Considerations

The power efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Although all dissipative

elements in circuits produce losses, four main sources

account for most of the losses in LT8390 circuits:

1. DC I2R losses. These arise from the resistances of the

MOSFETs, sensing resistor, inductor and PC board

traces and cause the efficiency to drop at high output

currents.

2. Transition loss. This loss arises from the brief amount

of time switch A or switch C spends in the saturated

region during switch node transitions. It depends upon

the input voltage, load current, driver strength and

MOSFET capacitance, among other factors.

3. INTVCC current. This is the sum of the MOSFET driver

and control currents.

4. CIN and COUT loss. The input capacitor has the dif-

ficult job of filtering the large RMS input current to the

regulator in buck region. The output capacitor has the

difficult job of filtering the large RMS output current in

boost region. Both CIN and COUT are required to have

low ESR to minimize the AC I2R loss and sufficient

capacitance to prevent the RMS current from causing

additional upstream losses in fuses or batteries.

5. Other losses. Schottky diode DBandDDareresponsible

for conduction losses during dead time and light load

conductionperiods.Inductorcorelossoccurspredomi-

nately at light loads. Switch A causes reverse recovery

currentlossinbuckregion,andswitchCcausesreverse

recovery current loss in boost region.

When making adjustments to improve efficiency, the

input current is the best indicator of changes in ef-

ficiency. If you make a change and the input current

decreases, then the efficiency has increased. If there is

no change in the input current, then there is no change

in efficiency.

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