LTC1703 LINER [Linear Technology], LTC1703 Datasheet - Page 24

• Manufacturer Part Number: LTC1703
• Description: Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1703
Now calculate the remaining values:
(K is a constant used in the calculations)
ƒ = chosen crossover frequency
G = 10
gain)
Type 2 Loop:
Type 3 Loop:
24
K Tan
R
R
K Tan
R
R
R
C
C
C
C
C
2
1
2
1
3
2
B
2
3
B
(GAIN/20)
C K
C K
2
2
2
2
2
V
V
2
2
K
OUT
OUT
V
R
V
K
2
–
REF
REF
K
1
1
BOOST
GKR
GR
C
C
1
2
1
–
1
BOOST
1
1
–
K R
–
–
(this converts GAIN in dB to G in absolute
1
2
1
R
R
V
V
1
4
1
1
REF
1
REF
3
U
45
45
U
W
U
CURRENT LIMIT PROGRAMMING
Programming the current limit on the LTC1703 is straight-
forward. The I
maximum allowable voltage drop across QB (the bottom
MOSFET) before the current limit circuit engages. The
voltage across QB is set by its on-resistance and the
current flowing in the inductor, which is the same as the
output current. The LTC1703 current limit circuit inverts
the voltage at I
voltage across QB, allowing the current limit to be set with
a positive voltage.
To set the current limit, calculate the expected voltage
drop across QB at the maximum desired current:
I
expected operating current, to allow for MOSFET R
changes with temperature. Setting I
maximum normal operating current is usually safe and will
adequately protect the power components if they are
chosen properly. The 100mV term is an approximate
factor that corrects for errors caused by ringing on the
switch node (illustrated in Figure 6). This factor will
change depending on the layout and the components
used, but 100mV is usually a good starting point. V
then programmed at the I
pull-up and an external resistor:
The resulting value of R
circuit to ensure that the I
MOSFET R
automobiles, and should be taken with a grain of salt.
Circuits that use very low values for R
be checked carefully, since small changes in R
cause large I
factor makes up a large percentage of the total V
value. If V
start up.
LIM
R
V
ILIM
PROG
should be chosen to be quite a bit higher than the
= V
PROG
DS(ON)
PROG
I
ILIM
LIM
MAX
MAX
is set too low, the LTC1703 may fail to
/10 A
changes when the 100mV correction
pin sets the current limit by setting the
specs are like horsepower ratings in
before comparing it with the negative
R
DS ON
ILIM
(
MAX
LIM
)
should be checked in an actual
circuit kicks in as expected.
pin using the internal 10 A
100
mV
LIM
IMAX
to 150% of the
(< 20k) should
IMAX
DROP
DS(ON)
PROG
can
is