LTC3709 datasheet(12/24 Pages) LINER

部件名	LTC3709

功能描述	Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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12

LTC3709

3709f

APPLICATIO S I FOR ATIO

The basic LTC3709 application circuit is shown on the

first page of this data sheet. External component selec-

tion is primarily determined by the maximum load cur-

rent and begins with the selection of the power MOSFET

switches and/or sense resistor. The inductor current is

determined by the RDS(ON) of the synchronous MOSFET

while the user has the option to use a sense resistor for

a more accurate current limiting. The desired amount of

ripple current and operating frequency largely deter-

mines 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) of the synchronous MOSFET or through

a sense resistance that appears between the SENSE– and

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

voltage applied to the VRNG pin and is equal to approxi-

mately VRNG/7.5. The current mode control loop will not

allow the inductor current valleys to exceed VRNG/(7.5 •

RSENSE). In practice, one should allow some margin for

variations in the LTC3709 and external component values.

A good guide for selecting the sense resistance for each

channel is:

R

V

I

SENSE

RNG

OUT MAX

=

2

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. Addition-

ally, 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 volt-

age is about 1.3 times this nominal value.

Connecting the SENSE+ and SENSE– Pins

The LTC3709 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 synchro-

nous MOSFET and ground. To measure the voltage across

this resistor, connect the SENSE+ pin to the source end of

the resistor and the SENSE– pin to the other end of the

resistor. The SENSE+ and SENSE– pin connections pro-

vide 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 in the Power MOSFET Selection section.

Power MOSFET Selection

The LTC3709 requires four external N-channel power

MOSFETs, two for the top (main) switches and two for the

bottom (synchronous) switches. Important parameters

for the power MOSFETs are the breakdown voltage

V(BR)DSS,thresholdvoltageV(GS)TH,on-resistanceRDS(ON),

reverse transfer capacitance CRSS and maximum current

IDS(MAX).

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

Consequently, logic-level threshold MOSFETs must be

used in LTC3709 applications. If the driver’s voltage is

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

MOSFETs should be used.

When the bottom MOSFETs are used as the current sense

elements, particular attention must be paid to their on-

resistance. MOSFET on-resistance is typically specified

with a maximum value RDS(ON)(MAX) at 25°C. In this case

additional margin is required to accommodate the rise in

MOSFET on-resistance with temperature:

R

DS ON MAX

SENSE

T

()(

) =

ρ

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

accounting for the significant variation in on-resistance

with temperature, typically about 0.4%/°C. Junction-to-

case temperature is about 20°C in most applications. For

a maximum junction temperature of 100°C, using a value

ρ100°C = 1.3 is reasonable (Figure 1).

类似零件编号 - LTC3709

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Linear Technology	LTC3709	326Kb / 24P	Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

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Linear Technology	LTC3709	326Kb / 24P	Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing
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LTC3709 数据表(PDF) 12 Page - Linear Technology

LTC3709 数据表(HTML) 12 Page - Linear Technology

类似零件编号 - LTC3709

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LTC3709 数据表(PDF) 12 Page - Linear Technology

LTC3709 数据表(HTML) 12 Page - Linear Technology

类似零件编号 - LTC3709

类似说明 - LTC3709

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