LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 12

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC3878
Figure 3 shows how R
for several common input votlages.
When designing for pseudo fi xed frequency, there is sys-
tematic error because the I
0.7V, not zero. This causes the I
proportional to (V
error increases as V
additional resistor R
pin to the 5.3V INTV
Minimum Off-Time and Dropout Operation
The minimum off-time, t
required for the LTC3878 to turn on the bottom MOSFET,
trip the current comparator and then turn off the bottom
MOSFET. This time is typically about 220ns. The minimum
off-time limit imposes a maximum duty cycle of t
(t
due to a drooping 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 4.
12
ON
R
V
IN MIN
ON
+ t
(
2
OFF(MIN)
=
1000
)
100
5 3
=
Figure 3. Switching Frequency vs R
100
.
V
V
V
0 7
OUT
). If the maximum duty cycle is reached,
OUT
V
.
OUT
– .
IN
= 1.5V
V
0 7
V
= 3.3V
t
OUT
IN
ON
– 0.7V) and not V
CC
ON2
decreases. To correct this error, an
V
= 5V
ON
supply.
+
R
can be connected from the I
t
R
t
ON
relates to switching frequency
ON
ON
OFF(MIN)
OFF MIN
ON
1000
(kΩ)
pin voltage is approximately
(
ON
)
, is the shortest time
current to be inversely
V
OUT
IN
= 12V
. The I
3878 F03
10000
ON
ON
current
ON
ON
/
Likewise, the maximum frequency of operation is deter-
mined by the fi xed on-time, t
t
duty factor by the nominal frequency of operation:
The LTC3878 is a PFM (pulse frequency mode) regula-
tor where pulse density is modulated, not pulse width.
Consequently, frequency increases with a load step and
decreases with a load release. The steady-state operating
frequency, f
allow for device tolerances and transient response.
Inductor Value Calculation
Given the desired input and output voltages, the induc-
tor value and operation frequency determine the ripple
current:
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.
OFF(MIN)
f
ΔI
MAX
Figure 4. Maximum Switching Frequncy vs Duty Cycle
L
=
. The fi xed on-time is determined by dividing the
=
⎛
⎜
⎝
f
V
OP
4
3
2
1
0
V
OP
IN
V
OUT
0
OUT
, should be set suffi ciently below f
•
•
L
f
OP
⎞
⎟
⎠
⎛
⎝ ⎜
0.25
+
DUTY CYCLE (V
1
– 1
t
OFF MIN
V
V
OUT
(
IN
0.50
ON
DROPOUT
REGION
⎞
⎠ ⎟
, and the minimum off-time,
)
OUT
[ ]
/V
Hz
0.75
IN
)
3878 F04
1
MAX
3878f
to