LT8705 datasheet(22/38 Pages) LINER

22 / 38 page

LTC3784

3784fb

For more information www.linear.com/LTC3784

applicaTions inForMaTion

exceeded. The INTVCC current, which is dominated by the

gate charge current, may be supplied by either the VBIAS

LDO or the EXTVCC LDO. When the voltage on the EXTVCC

pin is less than 4.8V, the VBIAS LDO is enabled. In this

case, power dissipation for the IC is highest and is equal

to VBIAS • IINTVCC. 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 equations given in Note 2 of the

Electrical Characteristics. For example, at 70°C ambient

temperature, the LTC3784 INTVCCcurrentislimitedtoless

than 21mA in the QFN package from a 60V VBIAS supply

when not using the EXTVCC supply:

TJ = 70°C + (21mA)(60V)(43°C/W) = 125°C

In an SSOP package, the INTVCC current is limited to

less than 11mA from a 60V supply when not using the

EXTVCC supply:

TJ = 70°C + (11mA)(60V)(80°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 (PLLIN/MODE

= INTVCC) at maximum VIN.

When the voltage applied to EXTVCC rises above 4.8V, the

VIN LDO is turned off and the EXTVCC LDO is enabled. The

EXTVCC LDO remains on as long as the voltage applied to

EXTVCC remains above 4.55V. The EXTVCC LDO attempts

to regulate the INTVCC voltage to 5.4V, so while EXTVCC

is less than 5.4V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 5.4V, up to an absolute maximum of 14V,

INTVCC is regulated to 5.4V.

Figure 6. Using the SS Pin to Program Soft-Start

LTC3784

CSS

SGND

3784 F06

INTVCC Regulators

The LTC3784 features two separate internal P-channel

low dropout linear regulators (LDO) that supply power at

the INTVCC pin from either the VBIAS supply pin or the

EXTVCC pin depending on the connection of the EXTVCC

pin. INTVCC powers the gate drivers and much of the

LTC3784’s internal circuitry. The VBIAS LDO and the

EXTVCC LDO regulate INTVCC to 5.4V. Each of these can

supplyatleast40mAandmustbebypassedtogroundwith

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 prevent interaction

between the channels.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC3784 to be

Figure 5. Setting Output Voltage

LTC3784

VFB

VOUT

3784 F05

LT8705 Datasheet(PDF) 22 Page - Linear Technology

LT8705 Datasheet(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Datasheet Download

Link URL