LTC3872 datasheet(6/20 Pages) LINER

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LTC3872

3872fa

OPERATIO

Main Control Loop

The LTC3872 is a No RSENSE constant frequency, current

mode controller for DC/DC boost, SEPIC and ﬂyback

converter applications. The LTC3872 is distinguished

from conventional current mode controllers because the

current control loop can be closed by sensing the voltage

drop across the power MOSFET switch or across a discrete

sense resistor, as shown in Figures 1 and 2. This No RSENSE

sensing technique improves efﬁciency, increases power

density and reduces the cost of the overall solution.

For circuit operation, please refer to the Block Diagram

of the IC and the Typical Application on the front page. In

normal operation, the power MOSFET is turned on when

the oscillator sets the RS latch and is turned off when the

current comparator resets the latch. The divided-down

output voltage is compared to an internal 1.2V reference by

the error ampliﬁer, which outputs an error signal at the ITH

pin. The voltage on the ITH pin sets the current comparator

input threshold. When the load current increases, a fall in

the FB voltage relative to the reference voltage causes the

ITH pin to rise, which causes the current comparator to

trip at a higher peak inductor current value. The average

inductor current will therefore rise until it equals the load

current, thereby maintaining output regulation.

The LTC3872 can be used either by sensing the voltage

drop across the power MOSFET or by connecting the SW

pin to a conventional sensing resistor in the source of the

power MOSFET. Sensing the voltage across the power

MOSFET maximizes converter efﬁciency and minimizes the

component count; the maximum rating for this pin, 60V,

allows MOSFET sensing in a wide output voltage range.

The RUN/SS pin controls whether the IC is enabled or is in

a low current shutdown state. With the RUN/SS pin below

0.85V, the chip is off and the input supply current is typi-

cally only 10μA. With an external capacitor connected to

the RUN/SS pin an optional external soft-start is enabled.

A 0.7μA trickle current will charge the capacitor, pulling

the RUN/SS pin above shutdown threshold and slowly

ramping RUN/SS to limit the VITH during start-up. Because

the noise on the SW pin could couple into the RUN/SS

pin, disrupting the trickle charge current that charges the

RUN/SS pin, a 1M resistor is recommended to pull-up the

RUN/SS pin when external soft-start is used. When RUN/SS

is driven by an external logic, a minimum of 2.75V logic is

recommended to allow the maximum ITH range.

Light Load Operation

Under very light load current conditions, the ITH pin volt-

age will be very close to the zero current level of 0.85V.

As the load current decreases further, an internal offset at

the current comparator input will assure that the current

comparator remains tripped (even at zero load current) and

the regulator will start to skip cycles, as it must, in order

to maintain regulation. This behavior allows the regulator

to maintain constant frequency down to very light loads,

resulting in low output ripple as well as low audible noise

and reduced RF interference, while providing high light

load efﬁciency.

Figure 1. SW Pin (Internal Sense Pin)

Connection for Maximum Efﬁciency

COUT

VSW

VOUT

VIN

GND

NGATE

GND

VIN

3872 F01

LTC3872

COUT

VSW

RSENSE

VOUT

VIN

GND

NGATE

GND

VIN

3872 F02

LTC3872

Figure 2. SW Pin (Internal Sense Pin)

Connection for Sensing Resistor

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