LTC3810-5
18
38105fd
Minimum Off-Time and Dropout Operation
The minimum off-time t
OFF(MIN)
is the smallest amount of
time that the LTC3810-5 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 250ns.
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:
V
IN(MIN)
=
V
OUT
t
ON
+
t
OFF(MIN)
t
ON
A plot of maximum duty cycle vs frequency is shown in
Figure 9.
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
Δ
I
L
=
V
OUT
f L
⎛
⎝⎜
⎞
⎠⎟
1
−
V
OUT
V
IN
⎛
⎝⎜
⎞
⎠⎟
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency 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
does not exceed a specified maximum, the inductance
should be chosen according to:
L
=
V
OUT
f
Δ
I
L(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
1
−
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ
®
cores. A variety of inductors designed for
high current, low voltage applications are available from
manufacturers such as Sumida, Panasonic, Coiltronics,
Coilcraft and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in the front page schematic
conducts during the dead time between the conduction
of the power MOSFET switches. It is intended to prevent
the body diode of the bottom MOSFET from turning on
and storing charge during the dead time, which can cause
a modest (about 1%) efficiency loss. The diode can be
rated for about one-half to one-fifth of the full load current
since it is on for only a fraction of the duty cycle. In order
for the diode to be effective, the inductance between it
C
VON
0.01µF
R
VON2
30k
R
VON1
100k
INTV
CC
5.5V
100k
V
ON
I
TH
LTC3810-5
38105 F08
2.0
1.5
1.0
0.5
0
0
0.25
0.50
0.75
38105 F09
1.0
DROPOUT
REGION
DUTY CYCLE (V
OUT
/V
IN
)
SWITCHING FREQUENCY (MHz)
applicaTions inForMaTion
Figure 9. Maximum Switching Frequency vs Duty Cycle
Figure 8. Correcting Frequency Shift with Load Current Changes