LTC3813 datasheet(13/32 Pages) LINER

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LTC3813

3813fb

The basic LTC3813 application circuit is shown on the ﬁrst

page of this data sheet. External component selection is

primarily determined by the maximum input voltage and

load current and begins with the selection of the sense

resistance and power MOSFET switches. The LTC3813

uses either a sense resistor or the on-resistance of the

synchronous power MOSFET for determining the induc-

tor current. The desired amount of ripple current and

operating frequency largely determines the inductor

value. Next, COUT is selected for its ability to handle the

large RMS current and with low enough ESR to meet the

output voltage ripple and transient speciﬁcation. Finally,

loop compensation components are selected to meet the

required transient/phase margin speciﬁcations.

Duty Cycle Considerations

For a boost converter, the duty cycle of the main switch is:

= 1

VIN

VOUT

;DMAX = 1

VIN(MIN)

VOUT

The maximum VOUT capability of the LTC3813 is inversely

proportional to the minimum desired operating frequency

and minimum off-time:

VOUT(MAX) =

VIN(MIN)

f MIN•tOFF(MIN)

100V

Maximum Sense Voltage and the VRNG Pin

The control circuit in the LTC3813 measures the input

current by using the RDS(ON) of the bottom MOSFET or

by using a sense resistor in the bottom MOSFET source,

so the output current needs to be reﬂected back to the

input in order to dimension the power MOSFET properly

and to choose the maximum sense voltage. Based on the

fact that, ideally, the output power is equal to the input

power, the maximum average input current and average

inductor current is:

IIN(MAX) =IL,AVG(MAX) =

IO(MAX)

1 DMAX

The current mode control loop will not allow the induc-

tor peak to exceed VSENSE(MAX)/RSENSE. In practice, one

should allow some margin for variations in the LTC3813

and external component values, and a good guide for

selecting the maximum sense voltage when VDS sensing

is used is:

VSENSE(MAX) =

1.7 • RDS(ON) •IO(MAX)

1 DMAX

VSENSE is set by the voltage applied to the VRNG pin. Once

VSENSE is chosen, the required VRNG voltage is calculated

to be:

VRNG = 5.78 • (VSENSE(MAX) + 0.026)

An external resistive divider from INTVCC can be used

to set the voltage of the VRNG pin between 0.5V and 2V

resulting in nominal sense voltages of 60mV to 320mV.

Additionally, the VRNG pin can be tied to SGND or INTVCC

in which case the nominal sense voltage defaults to 95mV

or 215mV, respectively.

Connecting the SENSE+ and SENSE– Pins

The LTC3813 can be used with or without a sense resis-

tor. When using a sense resistor, place it between the

source of the bottom MOSFET, M2, and PGND. Connect

the SENSE+ and SENSE– pins to the top and bottom of

the sense resistor. Using a sense resistor provides a well

deﬁned current limit, but adds cost and reduces efﬁciency.

Alternatively, one can eliminate the sense resistor and use

the bottom MOSFET as the current sense element by simply

connecting the SENSE+ pin to the lower MOSFET drain

and SENSE– pin to the MOSFET source. This improves

efﬁciency, but one must carefully choose the MOSFET

on-resistance, as discussed in the following section.

APPLICATIONS INFORMATION

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