LTC3100 datasheet(13/24 Pages) LINER

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LTC3100

3100fa

Current Sensing

Lossless current sensing converts the peak current signal

of the N-channel MOSFET switch into a voltage which

is summed with the internal slope compensation. The

summed signal is compared to the error ampliﬁer output to

provide a peak current control command for the PWM.

Current Limit

The current limit comparator shuts off the N-channel

MOSFET switch once its threshold is reached. Peak switch

current is no less than 700mA, independent of input or

output voltage, unless VOUT falls below 1V, in which case

the current limit is cut in half to minimize power dissipa-

tion into a short-circuit.

Slope Compensation

Current mode control requires the use of slope compensa-

tion to prevent subharmonic oscillations in the inductor

current waveform at high duty cycle operation. This is ac-

complished internally on the LTC3100 through the addition

of a compensating ramp to the current sense signal. The

LTC3100 performs current limiting prior to addition of the

slope compensation ramp and therefore achieves a peak

inductor current limit that is independent of duty cycle.

Zero Current Comparator

The zero current comparator monitors the boost inductor

current to the output and shuts off the synchronous rectiﬁer

once this current reduces to approximately 30mA. This

prevents the inductor current from reversing in polarity,

improving efﬁciency at light loads.

Synchronous Rectiﬁer

To control inrush current and to prevent the inductor

current from running away when VOUT is close to VIN,

the P-channel MOSFET synchronous rectiﬁer is only fully

enabled when VOUT > (VIN + 0.24V).

Anti-Ringing Control

The anti-ring circuitry connects a resistor across the

boost inductor to prevent high frequency ringing on the

SW pin during discontinuous current mode operation.

The ringing of the resonant circuit formed by L and CSW

(capacitance on SWBST pin) is low energy, but can cause

EMI radiation.

PGOOD Comparator

The PGBST pin is an open-drain output which indicates the

status of the boost converter output voltage. If the boost

output voltage falls 8% below the regulation voltage, the

PGBST open-drain output will pull low. The output voltage

must rise 3% above the falling threshold before the pull-

down will turn off. In addition, there is a 60μs (typical)

deglitching delay in order to prevent false trips due to

voltage transients on load steps. The PGBST output will

also pull low if the boost converter is disabled. The typical

PGBST pull-down switch resistance is 13Ω when VBST or

VINBST equals 3.3V.

Output Disconnect

The LTC3100 boost converter is designed to allow true

output disconnect by eliminating body diode conduction

of the internal P-channel MOSFET rectiﬁer. This allows for

VOUT to go to 0V during shutdown, drawing no current

from the input source. It also allows for inrush current

limiting at turn-on, minimizing surge currents seen by the

input supply. Note that to obtain the advantages of output

disconnect, there must not be an external Schottky diode

connected between SWBST and VBST. The output discon-

nect feature also allows VOUT to be pulled high without

any reverse current into the battery.

VIN > VOUT Operation

The LTC3100 boost converter will maintain voltage regu-

lation even when the input voltage is above the desired

output voltage. Note that the output current capability is

slightly reduced in this mode of operation. Refer to the

Typical Performance Characteristics section.

Burst Mode Operation (for Boost and Buck Converters)

Burst Mode operation for both converters can be enabled

or disabled using the MODE pin. If MODE is grounded,

Burst Mode operation is disabled for both the boost and

OPERATION

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