Datasheet

LTC3775
15
3775fa
Figure 8. Effi ciency in Pulse-Skipping/Forced Continuous Modes
Since the current limit comparator contains leading edge
blanking, an external RC fi lter is not required for proper
operation. However, an external fi lter can be designed by
adding a capacitor across the SENSE and I
LIMT
pins (C
F
in Figure 7a). The fi lter component should be placed close
to the SENSE and I
LIMT
pins.
If R
DS(ON)
sensing is employed, the Kelvin sense con-
nection should run from the SENSE pin and the R
ILIMT
resistor to the source and drain terminals of the top power
MOSFET respectively, as shown in Figure 7b. The external
RC fi lter should not be added since the source terminal
is switching.
The bottom side current limit threshold is set by the resis-
tor, R
ILIMB
, from the I
LIMB
pin to SGND and the I
LIMB
pin
10A pull-up current. The voltage at ILIMB is attenuated
5× internally before it is applied to the input of bottom
current limit comparator, CBLIM. This voltage must be
quiet. Connect R
ILIMB
from the I
LIMB
pin to a quiet ground
near the LTC3775 SGND pin. The other input of CBLIM is
connected to the SW pin. The SW pin is also shared with
the bottom gate driver and should be connected near the
drain of the bottom MOSFET, Q
B
.
Pulse-Skipping Mode
The LTC3775 can operate in one of two modes selectable
with the MODE/SYNC pin: pulse-skipping mode or forced
continuous mode. Pulse-skipping mode is selected when
increased effi ciency at light loads is desired, as shown in
Figure 8. In this mode, the bottom MOSFET is turned off
when inductor current reverses in order to minimize the
effi ciency loss due to reverse current fl ow. As the load
current decreases (see Figure 9), the duty cycle is reduced
to maintain regulation until the minimum on-time (50ns)
is reached. When the load decreases below this point,
the LTC3775 begins to skip cycles to maintain regulation.
This reduces the frequency and improves effi ciency by
minimizing gate charge losses.
In forced continuous mode, the bottom MOSFET is always
on when the top MOSFET is off, allowing the inductor cur-
rent to reverse at low currents. This mode is less effi cient
due to switching, but has the advantages of better transient
APPLICATIONS INFORMATION
R
SENSE
3775 F06
TO R
LIMIT
TO SENSE PIN
V
IN
TOP MOSFET
DRAIN
V
IN
C
IN
C
F
R
LIMIT
R
SENSE
3775 F07a
V
IN
LTC3775
I
LIMIT
SENSE
TG
SW
Figure 6. Kelvin SENSE Connection
for Topside Current Limiting Sensing
Figure 7a. External Filter for Topside Current Sensing
V
IN
C
IN
R
LIMIT
Q
T
3775 F07b
V
IN
LTC3775
I
LIMIT
SENSE
TG
SW
Figure 7b. Kelvin Connection for Topside R
DS(ON)
Sensing
LOAD CURRENT (A)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.01 1 10 100
3775 F08
0
0.1
V
IN
= 12V
V
OUT
= 1.2V
SW FREQ = 500kHz
FRONT PAGE CIRCUIT
PULSE-SKIPPING
MODE
CONTINUOUS
MODE