LT7101 datasheet(23/38 Pages) AD

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LT7101

Rev. 0

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APPLICATIONS INFORMATION

INTVCC that is at least 10x greater than the capacitor CBST

from BOOST to SW:

CVCC > 10 • CBST

Be careful to account for the voltage coefficient of ceramic

capacitors when choosing the value and case size. Many

ceramic capacitors lose 50% or more of their rated value

when used near their rated voltage.

For high VIN applications it is advantageous to tie EXTVCC

to VOUT (bootstrapping), as this will improve efficiency

and reduce power dissipation in the VIN LDO. This can

be done with any VOUT voltage between 3.3V and 40V.

Alternatively, the EXTVCC pin can be tied to any DC voltage

between 3.3V and 40V that is capable of delivering the

required INTVCC bias current, which varies with switch-

ing frequency and operating mode. At full-load operation,

which is the worst case, the INTVCC bias current is given

approximately by:

IINTVCC = 3.5mA+1nC 4+

VIN

⎛

⎝⎜

⎞

⎠⎟ • f

When EXTVCC is not present, the LDO timeout feature lim-

its the junction temperature rise due to the VIN LDO power

dissipation. See Soft-Start and LDO Regulator Timeout

section for more information.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC left open (or grounded). This will cause

INTVCC to be powered from the VIN LDO resulting in an

efficiency penalty at high input voltages.

2. EXTVCC connected directly to the output voltage VOUT.

This is the normal connection for a 3.3V to 40V regula-

tor and provides the highest efficiency.

3. EXTVCC connected to an external supply. If an external

supply is available in the 3.3V to 40V range, it may

be used to power EXTVCC providing it can supply the

required INTVCC current. Operating with EXTVCC > VIN

is allowed.

4. EXTVCC connected to an output-derived boost or

charge-pump network. For 2.5V and other low voltage

buck regulators, efficiency gains can still be realized by

connecting EXTVCC to an output-derived voltage that

has been boosted to greater than 3.05V.

Most applications will simply tie EXTVCC to VOUT for

high efficiency bootstrapping. In this configuration, with

Burst Mode operation selected, the no-load VIN current

in regulation can be calculated using:

IVIN = 1µA +

VOUT

0.8 • VIN

•

VOUT

6MΩ

+ 8µA

⎛

⎝⎜

⎞

⎠⎟

where RD is the total resistance of the feedback resistive

divider from VOUT to GND. In fixed output voltage mode,

where VOUT is programmed using VPRG1 and VPRG2, use

RD = VOUT/1.25µA. For adjustable VOUT mode (Figure 5),

use RD = R1 +R2.

Topside MOSFET Driver Supply (CBST)

The boost capacitor, CBST, on the Functional Diagram

is used to create a voltage rail above the applied input

voltage, VIN. Specifically, the boost capacitor is charged

through an internal MOSFET switch to a voltage equal

to approximately INTVCC each time the bottom power

MOSFET is turned on. The charge on this capacitor is then

used to supply the required current during the remainder

of the switching cycle. When the top MOSFET is turned

on, the BOOST pin voltage will be equal to approximately

VIN + 3.5V. For most applications a 0.1μF, X7R ceramic

capacitor will provide adequate performance.

The LT7101 also contains an internal charge pump that

supplies a small amount of current to the BOOST pin to

allow for continuous operation at 100% duty cycle. This

charge pump is adequate to support internal biasing

needs and to keep the top MOSFET fully enhanced. Note

that the total external leakage on the BOOST pin (includ-

ing the CBST capacitor leakage) must be less than 4µA to

ensure continuous operation at 100% duty cycle.

LT7101 Datasheet(PDF) 23 Page - Analog Devices

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