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參數(shù)資料
| 型號： | LTC3831IGN |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 500 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件頁數(shù)： | 20/20頁 |
| 文件大?。?/td> | 208K |
| 代理商： | LTC3831IGN |
LTC3831
9
3831fb
APPLICATIONS INFORMATION
back signal is greater than 3% above VREF. To prevent these
two comparators from triggering due to noise, the MIN and
MAX comparators’ response times are deliberately 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 dis-
ables both gate drivers if activated. If the chip junction
temperature 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 determined
by the required soft-start time. An internal 12μA current
source is included to charge CSS. 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 amplier from forcing the loop to
maximum duty cycle. The LTC3831 operates at low duty
cycle as the SS pin rises above 0.6V (VCOMP ≈ 1.2V). As
SS continues to rise, QSS turns off and the error amplier
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 control
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 IFB pin. CC compares the voltage at IFB to the
voltage at the IMAX pin. As the peak current rises, the
measured voltage across Q1 increases due to the drop
across the RDS(ON) of Q1. When the voltage at IFB drops
below IMAX, indicating that Q1’s drain current has exceeded
the maximum level, CC starts to pull current out of CSS,
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 IFB and
IMAX. 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 signicant overload conditions allow
the SS pin to reach a steady state, and the output remains
at a reduced voltage until the overload is removed. 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 RDS(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 RIMAX from the IMAX pin to the main VIN
supply at the drain of Q1. The value of RIMAX is determined
by:
RIMAX = (ILMAX)(RDS(ON)Q1)/IIMAX
where:
ILMAX = ILOAD + (IRIPPLE/2)
ILOAD = Maximum load current
IRIPPLE = Inductor ripple current
=
VIN –VOUT
() VOUT
()
fOSC
() LO
() VIN
()
fOSC = LTC3831 oscillator frequency = 200kHz
LO = Inductor value
RDS(ON)Q1 = On-resistance of Q1 at ILMAX
IIMAX = Internal 12μA sink current at IMAX
The RDS(ON) of Q1 usually increases with temperature.
To keep the current limit threshold constant, the internal
12μA sink current at IMAX is designed with a positive
temperature coefcient to provide rst order correction
for the temperature coefcient of RDS(ON)Q1.
In order for the current limit circuit to operate properly and
to obtain a reasonably accurate current limit threshold,
the IIMAX and IFB pins must be Kelvin sensed at Q1’s drain
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