LT1619 Linear Technology, LT1619 Datasheet - Page 9

• Manufacturer Part Number: LT1619
• Description: Low Voltage Current Mode PWM Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
V
current is the Burst Mode operation switch current thresh-
old, I
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
V
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 R
the sense resistor R
current I
C
C
operating range above V
voltage falls below V
Figure 5. Lowering Burst Mode Operation Current Limit
D(BURST)
BIAS
I
BIAS
of the current sense amplifier, which has a
.
= 120 A
OS
is added between the SENSE pin and
U
–
SENSE
SENSE
GND
U
B
+
5
4
B
. The corresponding switch
. Although Burst Mode opera-
AMPLIFIER
(Figure 5). The input bias
I
CURRENT
BIAS
SENSE
= 120 A
R
OS
W
R
1619 F05
SENSE
I
D
U
tolerance of 25% and is temperature stable, develops an
offset voltage at the sense input. The value of R
for non-Burst Mode operation can be obtained with the
expression:
where
For example, if I
Allowing for 25% and 30% variations in I
V
Choose R
operation. Lower values of R
100 ) 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.
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 V
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 R
C
SENSE(BURST)
S
I
V
R
I
R
has a corner frequency of:
BIAS
BIAS
SENSE(BURST)
OS
OS
= (1.25)(1.3)(83 )
R
R
OS
OS
120
OS
10
+ I
mV
= 137
V
L(MAX)
respectively:
SENSE(BURST)
A
SENSE
BIAS
= (Burst Mode operation peak switch
current, I
83
= 120 A and V
R
to completely disable Burst Mode
tends to be noisy. If the V
SENSE
= 40mV
D(BURST)
OS
(for example, 50
SENSE(BURST)
) • R
SENSE
LT1619
OS
BAIS
required
= 10mV:
OS
SENSE
1619fa
and
9
and
to