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LT1619ES8 Datasheet(PDF) 9 Page - Linear Technology |
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LT1619ES8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 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: R mV A OS ≥ µ =Ω 10 120 83 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 5 4 GND SENSE RSENSE ROS CURRENT SENSE AMPLIFIER IBIAS = 120µA 1619 F05 ID IBIAS = 120µA APPLICATIO S I FOR ATIO |
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