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LT8390A

8390afa

For more information www.linear.com/LT8390A

APPLICATIONS INFORMATION

Soft-Start and Short-Circuit Protection

As shown in Figure 8 and explained in the Operation sec-

tion, the SS pin can be used to program the output voltage

soft-start by connecting an external capacitor from the SS

pin to ground. The internal 12.5µA pull-up current charges

up the capacitor, creating a voltage ramp on the SS pin.

As the SS pin voltage rises linearly from 0.25V to 1V (and

beyond), the output voltage rises smoothly into its final

voltageregulation.Thesoft-starttimecanbecalculatedas:

tSS = 1V •

CSS

12.5µA

MakesuretheCSSisatleastfivetotentimeslargerthanthe

compensation capacitor on the VCpinforawell-controlled

output voltage soft-start. A 22nF 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 LT8390A can be set to three different

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

(499k), and keep-running (100k).

With a 100k resistor in keep-running mode, the LT8390A

continues switching normally and regulates the current

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

LT8390A stops switching until the EN/UVLO pin is pulled

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

the LT8390A 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

The LT8390A uses an internal transconductance error

amplifier, the output of which, VC, compensates the con-

trol loop. The external inductor, output capacitor, and the

compensation 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 2.2nF 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 LT8390A 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.

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