LTC4449 datasheet(18/48 Pages) AD

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LTC7872

Rev. 0

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At very light loads, the current comparator, ICMP, may

remain tripped for several cycles and force the external top

MOSFET to stay off for the same number of cycles (i.e.,

skipping-pulses). The inductor current is not allowed to

reverse (discontinuous operation). This mode, like forced

continuous operation, exhibits low output ripple as well

as low audio noise and reduced RF interference. It pro-

vides higher low current efficiency than forced continuous

mode, but not nearly as high as Burst Mode operation.

Boost Mode Light Load Current Operation (DCM/CCM)

In boost mode, the LTC7872 can be enabled to enter

constant-frequency discontinuous conduction mode or

forced continuous conduction mode. To select forced con-

tinuous operation, tie the MODE pin to SGND. To select

discontinuous conduction mode of operation, tie the

MODE pin to V5 or float it. In forced continuous operation,

the inductor current is allowed to reverse at light loads

or under large transient conditions. The inductor current

valley is determined by the voltage on the ITHHIGH pin,

just as in normal operation. In this mode, the efficiency

at light loads is lower. However, continuous mode has the

advantage of lower output ripple.

When the MODE pin is connected to V5 or floated, the

LTC7872 operates in discontinuous conduction mode at

light loads. At very light loads, the current comparator,

ICMP, may remain tripped for several cycles and force

the external top MOSFET to stay off for the same number

of cycles (i.e., skipping-pulses). The inductor current is

not allowed to reverse (discontinuous operation). This

mode, like forced continuous operation, exhibits low

output ripple as well as low audio noise and reduced RF

interference. It provides higher low current efficiency than

forced continuous mode.

The LTC7872 operation mode is summarized in Table 3.

Table 3. Operation Mode

MODE Pin

Buck Operation Mode

Boost Operation Mode

CCM

Float

Burst Mode Operation

DCM

VV5

DCM

Buck Mode Light Load Current Operation (DCM/CCM/

Burst Mode Operation)

In buck mode, the LTC7872 can be enabled to enter dis-

continuous conduction mode (DCM), forced continuous

conduction mode (CCM), or Burst Mode operation. To

select forced continuous operation, tie the MODE pin

to SGND. To select discontinuous conduction mode of

operation, tie the MODE pin to V5. To select Burst Mode

operation, float the MODE pin.

In forced continuous operation, the inductor current is

allowed to reverse at light loads or under large transient

conditions. The peak inductor current is determined by

the voltage on the ITHLOW pin, just as in normal operation.

In this mode, the efficiency at light loads is lower than

in DCM operation. However, continuous mode has the

advantages of lower output ripple and less interference

with audio circuitry.

When the LTC7872 is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-third of the maximum sense voltage even though the

voltage on the ITHLOW pin indicates a lower value. If the

average inductor current is higher than the load current,

the error amplifier, EA, will decrease the voltage on the

ITHLOW pin. When the ITHLOW voltage drops below 1.1V,

the internal sleep signal goes high (enabling sleep mode)

and both external MOSFETs are turned off.

In sleep mode, the load current is supplied by the output

capacitor. As the output voltage decreases, the EA’s out-

put begins to rise. When the output voltage drops enough,

the sleep signal goes low, and the controller resumes nor-

mal operation by turning on the top external MOSFET on

the next cycle of the internal oscillator. When a controller

is enabled for Burst Mode operation, the inductor cur-

rent is not allowed to reverse. The reverse current com-

parator (IREV) turns off the bottom external MOSFET just

before the inductor current reaches zero, preventing it

from reversing and going negative. Thus, the controller

operates in discontinuous conduction mode.

When the MODE pin is connected to V5, the LTC7872

operates in discontinuous conduction mode at light loads.

OPERATION

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