LTC3717_15 datasheet(9/20 Pages) LINER

Part #	LTC3717

Description	Wide Operating Range, No RSENSE Step-Down Controller for DDR/QDR Memory Termination

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Manufacturer	LINER [Linear Technology]

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LTC3717

sn3717 3717fs

APPLICATIO S I FOR ATIO

the load current. During LTC3717’s normal operation, the

duty cycles for the MOSFETs are:

D

V

D

VV

V

TOP

OUT

IN

BOT

IN

OUT

IN

=

–

The resulting power dissipation in the MOSFETs at maxi-

mum output current are:

PTOP = DTOP IOUT(MAX)2 ρT(TOP) RDS(ON)(MAX)

+ k VIN2 IOUT(MAX) CRSS f

PBOT = DBOT IOUT(MAX)2 ρT(BOT) RDS(ON)(MAX)

Both MOSFETs have I2R losses and the top MOSFET

includes an additional term for transition losses, which are

largest at high input voltages. The constant k = 1.7A–1 can

be used to estimate the amount of transition loss. The

bottom MOSFET losses are greatest when the bottom duty

cycle is near 100%, during a short-circuit or at high input

voltage.

JUNCTION TEMPERATURE (

°C)

–50

1.0

1.5

150

3717 F02

0.5

0

50

100

2.0

Figure 2. RDS(ON) vs. Temperature

Operating Frequency

The choice of operating frequency is a tradeoff between

efficiency and component size. Low frequency operation

improves efficiency by reducing MOSFET switching losses

but requires larger inductance and/or capacitance in order

to maintain low output ripple voltage.

The operating frequency of LTC3717 applications is deter-

mined implicitly by the one-shot timer that controls the

the LTC3717 and external component values and a good

guide for selecting the sense resistance is:

R

V

I

SENSE

RNG

OUT MAX

=

10 •

()

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 50mV to 200mV. Additionally,

the VRNG pin can be tied to SGND or INTVCC in which case

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

respectively. The maximum allowed sense voltage is about

1.3 times this nominal value for positive output current and

1.7 times the nominal value for negative output current.

Power MOSFET Selection

The LTC3717 requires two external N-channel power MOS-

FETs, 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 V(GS)TH, on-resistance RDS(ON), reverse transfer

capacitance CRSS and maximum current IDS(MAX).

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

Consequently, logic-level threshold MOSFETs must be

used in LTC3717 applications. If the input 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-

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 as shown in

Figure 2. For a maximum junction temperature of 100

°C,

using a value

ρT = 1.3 is reasonable.

The power dissipated by the top and bottom MOSFETs

strongly depends upon their respective duty cycles and

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