LTC7813 datasheet(20/28 Pages) AD

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LTC7878

Rev. 0

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APPLICATIONS INFORMATION

To prevent the maximum junction temperature from being

exceeded, the input supply current must be checked while

operating in continuous conduction mode at maximum

VIN. If the output voltage of the above example converter

is 24V and it is connected to EXTVCC pin. When the volt-

age applied to EXTVCC rises above 7.5V, the DRVCC linear

regulator from VIN is turned off and the linear regula-

tor from EXTVCC is turned on. Using EXTVCC allows the

MOSFET driver and control power to be derived from

the LTC7878’s switching regulator output during normal

operation and from the VIN when the output is out of reg-

ulation (e.g., start-up, short-circuit). Significant efficiency

and thermal gains can be realized by powering EXTVCC

from the 24V output, which reduces the junction tem-

perature in the previous example from 106.6°C to 57.6°C:

TJ = 25°C + (40mA)(24V)(34°C/W) = 57.6°C

Powering EXTVCC from the output can also provide

enough gate drive when VIN drops below 7V. This allows

a wider operating range for VIN after the controller start

into regulation. If both input and output voltages are high

voltages such as 40V, an external low voltage (8V ~ 10V)

bias supply is suggested to be used on the EXTVCC pin

for better efficiency and thermal performance.

Undervoltage Lockout

The LTC7878 has two ways that help protect the control-

ler in case of undervoltage conditions. A precision UVLO

comparator constantly monitors the DRVCC voltage to

ensure that an adequate gate-drive voltage is present.

It locks out the switching action when DRVCC is below

4.3V. To prevent oscillation when there is a disturbance

on the DRVCC, the UVLO comparator has 350mV of

precision hysteresis.

Another way to detect an undervoltage condition is to

monitor the VIN supply. Because the RUN pin has a pre-

cision turn-on reference of 1.2V, one can use a resis-

tor divider from VIN to turn on the IC when VIN is high

enough. An extra 4μA of current flows out of the RUN pin

once its voltage passes 1.22V. One can program the hys-

teresis of the run comparator by adjusting the values of

the resistive divider. To drive the non-logic level MOSFETs

with higher threshold voltages, it is suggested to set the

VIN UVLO higher than 7V using RUN pin for enough gate

drive voltage.

Soft Start-Up Function

When a capacitor is connected to the SS pin, a soft-start

current of 2.5μA starts to charge the capacitor. A soft-

start function is achieved by controlling the output ramp

voltage according to the ramp rate on the SS pin. Current

foldback is disabled during this phase to ensure smooth

soft-start. When the chip is in the shutdown state with its

RUN pin voltage below 1.22V, the SS pin is actively pulled

to ground. The soft-start range is defined to be the voltage

range from 0V to 1.0V on the SS pin. The total soft-start

time can be calculated as:

tSOFT-START = 1.0 •

CSS

2.5µA

Regardless of the mode selected by the MODE/ILIM pin,

the regulator will always start in pulse-skipping mode up

to SS = 1.0V.

Fault Conditions: Current Limit and Current Foldback

The maximum inductor current is inherently limited in a

current mode controller by the maximum sense voltage.

With 100mV current limit setup (MODE/ILIM tie to ground

and ISNSD short to ISNSP), maximum sense voltage and

the inductor DCR determine the maximum allowed induc-

tor peak current, which is:

IL(PEAK) =

100mV

DCR

To further limit current in the event of a short circuit to

ground, the LTC7878 includes foldback current limiting.

If the output falls by more than 40%, then the maximum

sense voltage is progressively lowered to about one-fifth

of its full value.

Efficiency Considerations

The percent 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

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