LTC3831 Datasheet, PDF (9/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831
APPLICATIO S I FOR ATIO
comparator forces the output to 0% duty cycle if the
feedback signal is greater than 3% above V_REF. To prevent
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 12µA
current source is included to charge C_SS. During power-
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_COMP ≈ 1.2V). As SS
continues to rise, Q_SS turns off and the error amplifier
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_FB pin. CC compares the voltage
at I_FB to the voltage at the I_MAX pin. As the peak current
rises, the measured voltage across Q1 increases due to the
drop across the R_DS(ON) of Q1. When the voltage at I_FB
drops below I_MAX, indicating that Q1’s drain current has
exceeded the maximum level, CC starts to pull current out
of C_SS, cutting the duty cycle and controlling the output
current level. The CC comparator pulls current out of the
SS pin in proportion to the voltage difference between I_FB
and I_MAX. Under minor overload conditions, the SS pin
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_DS(ON)
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_IMAX from the I_MAX pin to the main V_IN
supply at the drain of Q1. The value of R_IMAX is determined
by:
R_IMAX = (I_LMAX)(R_DS(ON)Q1)/I_IMAX
where:
I_LMAX = I_LOAD + (I_RIPPLE/2)
I_LOAD= Maximum load current
I_RIPPLE = Inductor ripple current
( ) ⁼
(^V_IN ^{– V}_OUT)(^V_OUT
f_{OSC (}L_O)(V_IN)
)
f_OSC = LTC3831 oscillator frequency = 200kHz
L_O = Inductor value
R_DS(ON)Q1 = On-resistance of Q1 at I_LMAX
I_IMAX = Internal 12µA sink current at I_MAX
The R_DS(ON) of Q1 usually increases with temperature. To
keep the current limit threshold constant, the internal
12µA sink current at I_MAX is designed with a positive
temperature coefficient to provide first order correction
for the temperature coefficient of R_DS(ON)Q1.
In order for the current limit circuit to operate properly and
to obtain a reasonably accurate current limit threshold, the
3831f
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