LTC3775 Linear Technology, LTC3775 Datasheet - Page 15

• Manufacturer Part Number: LTC3775
• Description: High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
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
in Figure 7a). The fi lter component should be placed close
to the SENSE and I
If R
nection should run from the SENSE pin and the R
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.
Figure 7b. Kelvin Connection for Topside R
DS(ON)
Figure 7a. External Filter for Topside Current Sensing
sensing is employed, the Kelvin sense con-
Figure 6. Kelvin SENSE Connection
for Topside Current Limiting Sensing
LTC3775
LTC3775
SENSE
TO R
TO SENSE PIN
SENSE
I
LIMT
LIMIT
I
LIMIT
V
TG
V
TG
LIMIT
IN
IN
pins.
3775 F07a
3775 F07b
TOP MOSFET
C
C
R
DRAIN
C
IN
F
R
LIMIT
IN
V
LIMIT
IN
R
3775 F06
SENSE
SW
SW
R
Q
SENSE
T
V
V
IN
IN
DS(ON)
LIMT
Sensing
pins (C
ILIMT
F
The bottom side current limit threshold is set by the resis-
tor, R
10μA 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
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
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
Figure 8. Effi ciency in Pulse-Skipping/Forced Continuous Modes
ILIMB
100
, from the I
90
80
70
60
50
40
30
20
10
0
0.01
V
V
SW FREQ = 500kHz
FRONT PAGE CIRCUIT
PULSE-SKIPPING
IN
OUT
ILIMB
= 12V
= 1.2V
0.1
MODE
LIMB
from the I
LOAD CURRENT (A)
CONTINUOUS
MODE
pin to SGND and the I
1
B
LIMB
.
10
pin to a quiet ground
LTC3775
3775 F08
100
LIMB
15
3775f
pin