LTC3831 LINER [Linear Technology], LTC3831 Datasheet - Page 9

• Manufacturer Part Number: LTC3831
• Description: High Power Synchronous Switching Regulator Controller for DDR Memory Termination
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
comparator forces the output to 0% duty cycle if the
feedback signal is greater than 3% above V
these two comparators from triggering due to noise, the
MIN and MAX comparators’ response times are deliber-
ately delayed by two to three microseconds. These two
comparators help prevent extreme output perturbations
with fast output load current transients, while allowing the
main feedback loop to be optimally compensated for
stability.
Thermal Shutdown
The LTC3831 has a thermal protection circuit that disables
both gate drivers if activated. If the chip junction tempera-
ture reaches 150 C, both TG and BG are pulled low. TG and
BG remain low until the junction temperature drops below
125 C, after which, the chip resumes normal operation.
Soft-Start and Current Limit
The LTC3831 includes a soft-start circuit that is used for
start-up and current limit operation. The SS pin requires
an external capacitor, CSS, to GND with the value deter-
mined by the required soft-start time. An internal 12 A
current source is included to charge C
up, the COMP pin is clamped to a diode drop (B-E junction
of QSS in the Block Diagram) above the voltage at the SS
pin. This prevents the error amplifier from forcing the loop
to maximum duty cycle. The LTC3831 operates at low duty
cycle as the SS pin rises above 0.6V (V
continues to rise, Q
takes over to regulate the output. The MIN comparator is
disabled during soft-start to prevent it from overriding the
soft-start function.
The LTC3831 includes yet another feedback loop to con-
trol operation in current limit. Just before every falling
edge of TG, the current comparator, CC, samples and
holds the voltage drop measured across the external
upper MOSFET, Q1, at the I
at I
rises, the measured voltage across Q1 increases due to the
drop across the R
drops below I
exceeded the maximum level, CC starts to pull current out
of C
FB
SS
to the voltage at the I
, cutting the duty cycle and controlling the output
MAX
, indicating that Q1’s drain current has
DS(ON)
U
SS
turns off and the error amplifier
of Q1. When the voltage at I
U
FB
MAX
pin. CC compares the voltage
pin. As the peak current
W
COMP
SS
. During power-
REF
1.2V). As SS
. To prevent
U
FB
current level. The CC comparator pulls current out of the
SS pin in proportion to the voltage difference between I
and I
falls gradually, creating a time delay before current limit
takes effect. Very short, mild overloads may not affect the
output voltage at all. More significant overload conditions
allow the SS pin to reach a steady state, and the output
remains at a reduced voltage until the overload is re-
moved. Serious overloads generate a large overdrive at
CC, allowing it to pull SS down quickly and preventing
damage to the output components. By using the R
of Q1 to measure the output current, the current limiting
circuit eliminates an expensive discrete sense resistor that
would otherwise be required. This helps minimize the
number of components in the high current path.
The current limit threshold can be set by connecting an
external resistor R
supply at the drain of Q1. The value of R
by:
where:
The R
keep the current limit threshold constant, the internal
12 A sink current at I
temperature coefficient to provide first order correction
for the temperature coefficient of R
In order for the current limit circuit to operate properly and
to obtain a reasonably accurate current limit threshold, the
R
I
I
I
f
L
R
I
LMAX
LOAD
RIPPLE
OSC
IMAX
O
IMAX
DS(ON)Q1
MAX
= Inductor value
DS(ON)
= LTC3831 oscillator frequency = 200kHz
= Maximum load current
= Internal 12 A sink current at I
= I
. Under minor overload conditions, the SS pin
= (I
= Inductor ripple current
LOAD
of Q1 usually increases with temperature. To
LMAX
= On-resistance of Q1 at I
V
IN
f
OSC
+ (I
–
)(R
IMAX
V
RIPPLE
OUT
DS(ON)Q1
L
O
from the I
MAX
/2)
V
V
OUT
IN
is designed with a positive
)/I
IMAX
MAX
DS(ON)Q1
pin to the main V
IMAX
LMAX
MAX
LTC3831
is determined
.
DS(ON)
9
3831f
FB
IN