LTC3255 datasheet(13/28 Pages) LINER

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LTC7820

7820fc

For more information www.linear.com/LTC7820

APPLICATIONS INFORMATION

If the LTC7820 divider input voltage is controlled by a

front end supply or hot swap controller and ramps up

slowly, the LTC7820 capacitor voltages are naturally bal-

anced. In this case the pre-balance and no load start-up

requirements are not necessary.

Voltage Doubler and Inverter Startup and Disconnect

In voltage doubler and inverter applications, LTC7820 can

startup without capacitor inrush charging current if the

input voltage is ramping slowly up from zero. As long as

the input voltage ramps up slow (in milliseconds), the

output voltage can track the input voltage and the voltage

difference between capacitors are always small resulting

in no huge inrush currents. The slew rate control of the

input voltage can be achieved by using a disconnect FET

at input or using hot swap controllers as shown in the

typicalapplicationsection.Differentfromvoltagedividers,

the voltage doubler and inverter applications have to start

up from zero input voltage every time, but they can start

up with heavy load currents directly.

Note that voltage divider applications can also startup with

a slow ramping input voltage from zero to the steady state

operation if there is a hot swap in front of the LTC7820,

(pre-balance is not required).

Overcurrent Protection

The LTC7820 provides overcurrent protection through

a sensing resistor placed on the high voltage side. A

precision rail to rail comparator monitors the differential

voltage between the ISENSE+ pin and the ISENSE– pin which

are Kelvin connected to a sensing resistor. Whenever the

ISENSE+ pin voltage is 50mV higher than the ISENSE– pin

voltage,anovercurrentfaultistriggeredandtheFAULTpin

is pulled down to ground. At the same time the LTC7820

stops switching and starts retry mode based on the timer

pin setup. The overcurrent fault will be cleared when the

timer pin voltage reaches 4V and the voltage across the

sensing resistor is less than 50mV. The current through

the sensing resistor is a pulse current during charging/

discharging of the flying capacitors, which may result a

voltage higher than the 50mV threshold at heavy loads.

To prevent the inrush current from falsely triggering the

overcurrent protection, an RC filter is required at the

ISENSE+ pin and ISENSE–. The RC filter timer constant has

to be larger than a switching period. Typically a 100Ω and

0.1µF filter is good for most of applications. Due to the

current flowing into the ISENSE+ pin, the resistor of the

RC filter has to be placed at the ISENSE– pin. ISENSE+ pin

needs to be connected to the sensing resistor directly. The

current limit can be selected by choosing different sense

resistor values. For example, the 10mΩ sense resistor

sets current limit at 50mV/10mΩ = 5A ideally. Due to the

switching ripple, the actual current limit is always lower

than the ideal case. In real circuits, the current limit is

around 4.2A with 0.1µF/100Ω filter and 200kHz switch-

ing frequency. The LTspice® simulation tool can be used

to quantify the switching ripple.

The overcurrent protection can also be used in doubler

and inverter applications for overcurrent and short-circuit

conditions at both startup and steady state operation. If

over current protection is not used, short the ISENSE+ pin

and the ISENSE– pin together and connect them to the drain

of the top MOSFET M1.

Window Comparator Programming

Innormaloperation,VLOW_SENSEvoltageshouldbealways

close to half of the VHIGH_SENSE voltage. A floating win-

dow comparator monitors the voltage on the VLOW_SENSE

pin and compares it with VHIGH_SENSE/2. The hysteresis

window voltage can be programmed and is equal to the

voltage at the HYS_PRGM pin. There is a precision 10µA

current flowing out of HYS_PRGM pin. A single resistor

from HYS_PRGM pin to GND sets the HYS_PRGM pin

voltage, which equals the resistor value multiplied by

10µA current (e.g. the voltage is 1V with a 100k resistor

from the HYS_PRGM pin to GND). With a 100k resistor

on the HYS_PRGM pin, the VHIGH_SENSE/2 voltage has to

be within a (VLOW_SENSE ±1V) window during startup and

normal operation, otherwise a fault is triggered and the

LTC7820 stops switching.

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