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LTC3774

Dual, Multiphase Current Mode Synchronous Controller

Linear Technology 

LTC3774

APPLICATIONS INFORMATION

The Typical Application on the first page of this data sheet

is a basic LTC3774 application circuit. The LTC3774 is

designed and optimized for use with a very low DCR

value by utilizing a novel approach to reduce the noise

sensitivity of the sensing signal by a factor of 14dB. DCR

sensing is becoming popular because it saves expensive

current sensing resistors and is more power efficient,

especially in high current applications. However, as the

DCR value drops below 1mΩ, the signal-to-noise ratio

is low and current sensing is difficult. LTC3774 uses an

LTC proprietary technique to solve this issue. In general,

external component selection is driven by the load require-

ment, and begins with the DCR and inductor value. Next,

power MOSFETs are selected. Finally, input and output

capacitors are selected.

Current Limit Programming

The ILIM pin is a 5-level logic input which sets the maxi-

mum current limit of the controller. When ILIM is either

grounded, floated or tied to INTVCC, the typical value for

the maximum current sense threshold will be 10mV,

20mV or 30mV, respectively. Setting ILIM to one-fourth

INTVCC and three-fourths INTVCC for maximum current

sense thresholds of 15mV and 25mV. Setting ILIM using

a resistor divider off of INTVCC will allow the maximum

current sense threshold setting to not change when the

5.5V LDO is in dropout at start-up. Please note that the

ILIM pin has an internal 500k pull-down to GND and a 500k

pull-up to INTVCC.

Which setting should be used? For the best current limit

accuracy, use the highest setting that is applicable to the

output requirements.

SNSD+, SNSA+ and SNS– Pins

The SNSA+ and SNS– pins are the inputs to the current

comparators, while the SNSD+ pin is the input of an

internal amplifier. The operating input voltage range is

0V to 3.5V for all three sense pins. All the positive sense

pins that are connected to the current comparator or the

amplifier are high impedance with input bias currents

of less than 1µA, but there is also a resistance of about

300k from the SNS– pin to ground. The SNS– should

be connected directly to VOUT. The SNSD+ pin connects

to the filter that has a R1•C1 time constant matched to

L/DCR of the inductor. The SNSA+ pin is connected to

the second filter with the time constant one-fifth that

of R1•C1. Care must be taken not to float these pins

during normal operation. Filter components, especially

capacitors, must be placed close to the LTC3774, and

the sense lines should run close together to a Kelvin con-

nection underneath the current sense element (Figure 3).

Because the LTC3774 is designed to be used with a very

low DCR value to sense inductor current, without proper

care, the parasitic resistance, capacitance and inductance

will degrade the current sense signal integrity, making

the programmed current limit unpredictable. As shown

in Figure 4, resistors R1 and R2 are placed close to the

output inductor and capacitors C1 and C2 are close to

the IC pins to prevent noise coupling to the sense signal.

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR

COUT

3774 F03

Figure 3. Sense Lines Placement with Inductor DCR

For more information www.linear.com/LTC3774

3774f

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