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LT1619

1619fa

Diode

Schottky diodes are recommended for low output voltage

applications because of their low forward voltage. Since

Schottky diodes have negligible stored charge, charge

dumping loss is also reduced. The reverse breakdown

voltage of the diode should exceed the maximum reverse

voltage stress of the topology used. The diode should also

be able to carry the peak diode current with acceptable

foward voltage. For the boost converter in Figure 1, the

peak inductor current is approximately 5A. A Motorola

MBRD835 is used due to its low forward voltage.

Lowering Burst Mode Operation Current Limit

The LT1619 automatically enters Burst Mode operation as

VC voltage falls below VB. The corresponding switch

current is the Burst Mode operation switch current thresh-

old, ID(BURST).

The effective Burst Mode operation current threshold can

be lowered by adding an offset to the input of the current

sense amplifier so that the switch current appears higher

to the PWM comparator. This has the effect of shifting the

VC operating range above VB. Although Burst Mode opera-

tion is not entirely disabled, the peak switch current before

entering Burst Mode operation is greatly reduced due to

the offset of the current sense amplifier. The peak switch

current is also determined by the current sense amplifier

blanking.

To lower the Burst Mode operation current sense thresh-

old, a resistor ROS is added between the SENSE pin and

the sense resistor RSENSE (Figure 5). The input bias

current IBIAS of the current sense amplifier, which has a

tolerance of

±25% and is temperature stable, develops an

offset voltage at the sense input. The value of ROS required

for non-Burst Mode operation can be obtained with the

expression:

IBIASROS ≥ VSENSE(BURST)

where

VSENSE(BURST) = (Burst Mode operation peak switch

current, ID(BURST)) • RSENSE

For example, if IBIAS = 120µA and VSENSE(BURST) = 10mV:

OS ≥

=Ω

120

Allowing for 25% and 30% variations in IBAIS and

VSENSE(BURST) respectively:

ROS = (1.25)(1.3)(83Ω)

Choose ROS = 137Ω to completely disable Burst Mode

operation. Lower values of ROS (for example, 50Ω to

100

Ω) can be used to lower the effective Burst Mode

current limit.

The value of the sense resistor is then adjusted to compen-

sate for the reduced full-scale sense voltage.

IBIASROS + IL(MAX)RSENSE = 40mV

Filtering Current Sense Signal

In a current mode converter, the current sense circuit

senses the switch current and terminates the switch

conduction. In the LT1619, the current sense amplifier

has a full-scale input voltage range from the ground to the

current limit threshold (53mV). Due to high speed switch-

ing transients and parasitic trace inductances, the current

sense signal VSENSE tends to be noisy. If the VSENSE

switching transient is excessive, the current sense ampli-

fier will amplify the spurious transient instead, resulting in

jittery operation. In situations where the internal leading

edge blanking is inadequate, a lowpass filter (Figure 6)

with corner frequency about 5 times the switching

frequency can be used to further attenuate high speed

switching transients. In Figure 6 the lowpass filter ROS and

CS has a corner frequency of:

Figure 5. Lowering Burst Mode Operation Current Limit

GND

SENSE

RSENSE

ROS

CURRENT

SENSE

AMPLIFIER

IBIAS = 120µA

1619 F05

IBIAS = 120µA

APPLICATIO S I FOR ATIO

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