Datasheet

LTC3823
13
3823fd
When there is no R
ON
resistor connected to the I
ON
pin,
the on-time t
ON
is theoretically infi nite, which in turn could
damage the converter. To prevent this, the LTC3823 detects
this fault condition and provides a minimum I
ON
current
of 5μA to 10μA.
Changes in the load current magnitude will cause fre-
quency shift. Parasitic resistance in the MOSFET switches
and inductor reduce the effective voltage across the
inductance, resulting in increased duty cycle as the load
current increases. By lengthening the on-time slightly as
current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
TH
pin to the V
ON
pin and V
OUT
. The values
required will depend on the parasitic resistances in the
specifi c application. A good starting point is to feed about
25% of the voltage change at the I
TH
pin to the V
ON
pin
as shown in Figure 3a. Place capacitance on the V
ON
pin
to fi lter out the I
TH
variations at the switching frequency.
The resistor load on I
TH
reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 3b.
MOSFET back off. This time is generally about 280ns.
The minimum off-time limit imposes a maximum duty
cycle of t
ON
/(t
ON
+ t
OFF(MIN)
). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
VV
tt
t
IN MIN OUT
ON OFF MIN
ON
()
()
=
+
A plot of maximum duty cycle vs frequency is shown in
Figure 4.
APPLICATIONS INFORMATION
C
VON
0.01μF
R
VON2
100k
R
VON1
30k
C
C
V
OUT
R
C
(3a)
(3b)
V
ON
I
TH
LTC3823
C
VON
0.01μF
R
VON2
10k
Q1
2N5087
R
VON1
3k
10k
C
C
3823 F03
V
OUT
INTV
CC
R
C
V
ON
I
TH
LTC3823
Figure 3. Correcting Frequency Shift with Load Current Changes
Minimum Off-Time and Dropout Operation
The minimum off-time t
OFF(MIN)
is the smallest amount of
time that the LTC3823 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
2.0
1.5
1.0
0.5
0
0 0.25 0.50 0.75
3823 F04
1.0
DROPOUT
REGION
DUTY CYCLE (V
OUT
/V
IN
)
SWITCHING FREQUENCY (MHz)
Figure 4. Maximum Switching Frequency vs Duty Cycle
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
ΔI
V
fL
V
V
L
OUT OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
−
⎛
⎝
⎜
⎞
⎠
⎟
•
1
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest effi ciency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, effi ciency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current