Datasheet

LTC3878
12
3878fa
applicaTions inForMaTion
Figure 3 shows how R
ON
relates to switching frequency
for several common output voltages.
When designing for pseudo fixed frequency, there is sys-
tematic error because the I
ON
pin voltage is approximately
0.7V, not zero. This causes the I
ON
current to be inversely
proportional to (V
IN
– 0.7V) and not V
IN
. The I
ON
current
error increases as V
IN
decreases. To correct this error, an
additional resistor R
ON2
can be connected from the I
ON
pin to the 5.3V INTV
CC
supply.
R
V V
V
R
ON ON2
5 3 0 7
0 7
=
. – .
.
Likewise, the maximum frequency of operation is deter-
mined by the fixed on-time, t
ON
, and the minimum off-time,
t
OFF(MIN)
. The fixed on-time is determined by dividing the
duty factor by the nominal frequency of operation:
f
V
V f
t
Hz
MAX
OUT
IN OP
OFF MIN
=
+
1
•
[ ]
( )
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
OP
, should be set sufficiently below f
MAX
to
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:
∆I
V
f L
V
V
L
OUT
OP
OUT
IN
=












•
–1
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 trade-off between
component size, efficiency and operating frequency.
Figure 4. Maximum Switching Frequency vs Duty Cycle
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3878 F03
V
OUT
= 1.5V
V
OUT
= 12V
V
OUT
= 3.3V
V
OUT
= 5V
Figure 3. Switching Frequency vs R
ON
Minimum Off-Time and Dropout Operation
The minimum off-time, t
OFF(MIN)
, is the shortest time
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
ON
/
(t
ON
+ t
OFF(MIN)
). If the maximum duty cycle is reached,
due to a drooping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
V V
t t
t
IN MIN OUT
ON OFF MIN
ON
( )
( )
=
+
A plot of maximum duty cycle vs. frequency is shown in
Figure 4.
DUTY CYCLE (V
OUT
/V
IN
)
DROPOUT
REGION
SWITCHING FREQUENCY (MHz)
2
3
3878 F04
1
0
0 0.25 0.50 0.75 1
4