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LTC3708

3708fb

APPLICATIONS INFORMATION

The basic LTC3708 application circuit is shown on the ﬁrst

page of this data sheet. External component selection is

primarily determined by the maximum load current and

begins with the selection of the power MOSFET switches

and/or sense resistor. For the LTC3708, the inductor cur-

rent is determined by the RDS(ON) of the synchronous

MOSFET or by a sense resistor when the user opts for

more accurate current sensing. The desired amount of

ripple current and operating frequency largely determines

the inductor value. Finally, CIN is selected for its ability to

handle the large RMS current into the converter and COUT

is chosen with low enough ESR to meet the output voltage

ripple speciﬁcation.

Maximum Sense Voltage and VRNG Pin

Inductor current is determined by measuring the voltage

across the RDS(ON) ofthesynchronousMOSFETorthrough

a sense resistor that appears between the SENSE+ and

SENSE– pins. The maximum sense voltage is set by the

voltage applied to the VRNGpinandisequaltoapproximately

VRNG/7. The current mode control loop will not allow the

inductor current valleys to exceed VRNG/(7 • RSENSE). In

practice, one should allow some margin for variations in

the LTC3708 and external component values. A good guide

for selecting the sense resistance is:

SENSE

RNG

OUT MAX

10 •

()

The voltage of the VRNG pin can be set using an external

resistive divider from VCC between 0.5V and 2V, resulting

in nominal sense voltages of 50mV to 200mV. Additionally,

the VRNG pin can be tied to ground or VCC, in which case

the nominal sense voltage defaults to 70mV or 140mV,

respectively. The maximum allowed sense voltage is about

1.4 times this nominal value.

Connecting the SENSE+ and SENSE– Pins

The LTC3708 provides the user with an optional method

to sense current through a sense resistor instead of using

the RDS(ON) of the synchronous MOSFET. When using a

sense resistor, it is placed between the source of the syn-

chronous MOSFET and ground. To measure the voltage

across this resistor, connect the SENSE+ pin to the source

of the synchronous MOSFET and the SENSE– pin to the

other end of the resistor. The SENSE+ and SENSE– pins

provide the Kelvin connections, ensuring accurate voltage

measurement across the resistor. Using a sense resistor

provides a well-deﬁned current limit, but adds cost and

reduces efﬁciency. Alternatively, one can use the synchro-

nous MOSFET as the current sense element by simply

connecting the SENSE+ pin to the switch node SW and

the SENSE– pin to the source of the synchronous MOSFET,

eliminating the sense resistor. This improves efﬁciency,

but one must carefully choose the MOSFET on-resistance

as discussed below.

Power MOSFET Selection

Each output stage of the LTC3708 requires two external

N-channel power MOSFETs, one for the top (main) switch

and one for the bottom (synchronous) switch. Important

parameters for the power MOSFETs are the breakdown

voltage V(BR)DSS, threshold voltage VGS(TH), on-resistance

RDS(ON),reversetransfercapacitance,CRSS,andmaximum

current, IDS(MAX).

The gate drive voltage is set by the 5V DRVCC supply.

Consequently, logic-level threshold MOSFETs must be

used in LTC3708 applications. If the driver’s voltage is

expected to drop below 5V, then sub-logic level threshold

MOSFETs should be considered.

When the bottom MOSFET is used as the current sense

element, particular attention must be paid to its on-re-

sistance. MOSFET on-resistance is typically speciﬁed

with a maximum value RDS(ON)(MAX) at 25°C. Additional

margin is required to accommodate the rise in MOSFET

on-resistance with temperature:

DS ON MAX

SENSE

()(

)

The

ρT term is a normalization factor (unity at 25°C) ac-

counting for the signiﬁcant variation in on-resistance with

temperature, typically about 0.4%/°C. For a maximum

junction temperature of 100°C, using a value

ρ100°C =

1.3 is reasonable (see Figure 3).

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