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LTC3787

3787fc

APPLICATIONS INFORMATION

Setting Output Voltage

The LTC3787 output voltage is set by an external feedback

resistor divider carefully placed across the output, as shown

in Figure 5. The regulated output voltage is determined by:

OUT = 1.2V

⎛

⎝⎜

⎞

⎠⎟

Great care should be taken to route the VFB line away

from noise sources, such as the inductor or the SW line.

Also keep the VFB node as small as possible to avoid

noise pickup.

INTVCC Regulators

The LTC3787 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

LTC3787’s internal circuitry. The VBIAS LDO and the

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

supply at least 50mA 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 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 LTC3787 to be

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 3 of the

Electrical Characteristics. For example, at 70°C ambient

temperature, the LTC3787 INTVCC current is limited to less

than 32mA in the QFN package from a 40V VBIAS supply

when not using the EXTVCC supply:

TJ = 70°C + (32mA)(40V)(43°C/W) = 125°C

In an SSOP package, the INTVCC current is limited to

less than 15mA from a 40V supply when not using the

EXTVCC supply:

TJ = 70°C + (15mA)(40V)(90°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

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

LTC3787

CSS

SGND

3787 F06

Soft-Start (SS Pin)

The start-up of VOUT is controlled by the voltage on the

SS pin. When the voltage on the SS pin is less than the

internal 1.2V reference, the LTC3787 regulates the VFB

pin voltage to the voltage on the SS pin instead of 1.2V.

Soft-start is enabled by simply connecting a capacitor from

the SS pin to ground, as shown in Figure 6. An internal

10μA current source charges the capacitor, providing a

linear ramping voltage at the SS pin. The LTC3787 will

regulate the VFB pin (and hence, VOUT) according to the

voltage on the SS pin, allowing VOUT to rise smoothly

from VIN to its final regulated value. The total soft-start

time will be approximately:

SS = CSS •

1.2V

10µA

Figure 5. Setting Output Voltage

LTC3787

VFB

VOUT

3787 F05

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