ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 12

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3711
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I
shown in Figure 4a. Place capacitance on the V
filter out the I
resistor load on I
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 4b.
Minimum Off-time and Dropout Operation
The minimum off-time t
time that the LTC3711 is capable of turning on the bottom
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
due to a dropping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
A plot of maximum duty cycle vs frequency is shown in
Figure 5.
12
ON
/(t
V
IN MIN
ON
(
+ t
)
TH
OFF(MIN)
V
OUT
TH
V
pin to the V
OUT
variations at the switching frequency. The
R
R
TH
VON1
30k
VON2
100k
). If the maximum duty cycle is reached,
U
t
reduces the DC gain of the error amp
ON
OFF(MIN)
C
R
C
t
U
C
t
ON
(4a)
OFF MIN
ON
(
pin and V
Figure 4. Correcting Frequency Shift with Load Current Changes
is the smallest amount of
C
0.01 F
VON
TH
)
W
pin to the V
V
I
TH
LTC3711
ON
OUT
3711 F04a
. The values
U
ON
ON
pin as
pin to
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
Lower ripple current reduces cores 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
occurs at the highest V
does not exceed a specified maximum, the inductance
should be chosen according to:
Figure 5. Maximum Switching Frequency vs Duty Cycle
I
L
2.0
1.5
1.0
0.5
INTV
V
0
V
OUT
fL
OUT
0
CC
2N5087
R
10k
VON1
3k
1
0.25
DUTY CYCLE (V
Q1
OUT(MAX)
V
IN
R
10k
V
OUT
VON2
. To guarantee that ripple current
IN
(4b)
0.50
OUT
. The largest ripple current
DROPOUT
REGION
/V
C
0.01 F
C
VON
C
IN
R
0.75
C
)
V
I
TH
LTC3711
ON
3711 F05
1.0
3711 F04b
3711f