LTC3880 datasheet(25/50 Pages) LINER

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LTC3839

3839fa

APPLICATIONS INFORMATION

For high switching frequencies, reducing output ripple and

better EMI filtering may require small value capacitors that

have low ESL (and correspondingly higher self-resonant

frequencies) to be placed in parallel with larger value

capacitors that have higher ESL. This will ensure good

noise and EMI filtering in the entire frequency spectrum

of interest. Even though ceramic capacitors generally

have good high frequency performance, small ceramic

capacitors may still have to be parallel connected with

large ones to optimize performance.

High performance through-hole capacitors may also be

used, but an additional ceramic capacitor in parallel is

recommended to reduce the effect of their lead inductance.

Remember also to place high frequency decoupling capaci-

tors as close as possible to the power pins of the load.

Top MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor, CB, connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFET. This capacitor is charged through diode DB from

DRVCC when the switch node is low. When the top MOSFET

turns on, the switch node rises to VIN and the BOOST pin

rises to approximately VIN + INTVCC. The boost capacitor

needs to store approximately 100 times the gate charge

required by the top MOSFET. In most applications a 0.1μF

to 0.47μF, X5R or X7R dielectric capacitor is adequate. It

is recommended that the BOOST capacitor be no larger

than 10% of the DRVCC capacitor, CDRVCC, to ensure that

the CDRVCC can supply the upper MOSFET gate charge

and BOOST capacitor under all operating conditions. Vari-

able frequency in response to load steps offers superior

transient performance but requires higher instantaneous

gate drive. Gate charge demands are greatest in high

frequency low duty factor applications under high load

steps and at start-up.

DRVCC Regulator and EXTVCC Power

The LTC3839 features a PMOS low dropout (LDO) linear

regulator that supplies power to DRVCCfromtheVINsupply.

The LDO regulates its output at the DRVCC1 pin to 5.3V.

The LDO can supply a maximum current of 100mA and

must be bypassed to ground with a minimum of 4.7μF

ceramic capacitor. Good bypassing is needed to supply

the high transient currents required by the MOSFET gate

drivers and to minimize interaction between the channels.

High input voltage applications in which large MOSFETs are

being driven at high frequencies may cause the maximum

junction temperature rating for the LTC3839 to be exceeded,

especially if the LDO is active and provides DRVCC. Power

dissipation for the IC in this case is highest and is ap-

proximately equal to VIN • IDRVCC. The gate charge current

is dependent on operating frequency as discussed in the

Efficiency Considerations section. The junction temperature

can be estimated by using the equation given in Note 2

of the Electrical Characteristics. For example, when using

the LDO, LTC3839’s DRVCC current is limited to less than

32mA from a 38V supply at TA = 70°C:

TJ = 70°C + (32mA)(38V)(44°C/W) = 125°C

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.

When the voltage applied to the EXTVCC pin rises above

the switchover voltage (typically 4.6V), the VIN LDO is

turned off and the EXTVCC is connected to DRVCC2 pin with

an internal switch. This switch remains on as long as the

voltage applied to EXTVCC remains above the hysteresis

(around 200mV) below the switchover voltage. Using

EXTVCC allows the MOSFET driver and control power to

be derived from the LTC3839’s switching regulator output

VOUT during normal operation and from the LDO when the

output is out of regulation (e.g., start up, short circuit). If

more current is required through the EXTVCC than is speci-

fied, an external Schottky diode can be added between the

EXTVCC and DRVCC pins. Do not apply more than 6V to the

EXTVCC pin and make sure that EXTVCC is less than VIN.

Significant efficiency and thermal gains can be realized

by powering DRVCC from the switching converter output,

since the VIN current resulting from the driver and control

currents will be scaled by a factor of (Duty Cycle)/(Switcher

Efficiency).

Tying the EXTVCC pin to a 5V supply reduces the junction

temperature in the previous example from 125°C to:

TJ = 70°C + (32mA)(5V)(44°C/W) = 77°C

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