LTC3831-1 Linear Technology, LTC3831-1 Datasheet - Page 10

• Manufacturer Part Number: LTC3831-1
• Description: High Power Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

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LTC3831-1
APPLICATIO S I FOR ATIO
actual current limit to be greater than the desired current
limit set point. Due to switching noise and variation of
R
accurate. The current limiting circuitry is primarily meant
to prevent damage to the power supply circuitry during
fault conditions. The exact current level where the limiting
circuit begins to take effect will vary from unit to unit as the
R
current, this can give a 65% variation on the current limit
threshold.
The R
low. This occurs during power up, when PV
up. To prevent the high R
limit, the LTC3831-1 disables the current limit circuit if
PV
operation of the current limit circuit, PV
least 2V above V
when TG is low, allowing the use of an external charge
pump to power PV
Oscillator Frequency
The LTC3831-1 includes an onboard current controlled
oscillator that typically free-runs at 300kHz. The oscillator
frequency can be adjusted by forcing current into or out of
the FREQSET pin. With the pin floating, the oscillator runs
at about 300kHz. Every additional 1 A of current into/out
of the FREQSET pin decreases/increases the frequency by
10kHz. The pin is internally servoed to 1.265V. The fre-
quency can be estimated as:
10
LTC3831-1
40% and with 25% variation on the LTC3831-1’s I
DS(ON)
DS(ON)
CC1
f
DS(ON)
CC
300
is less than 2V above V
, the actual current limit trip point is not highly
of Q1 varies. Typically, R
+
–
kHz
is high if the V
Figure 2. Current Limit Setting
12 A
1 265
CC
.
CC1
U
when TG is high. PV
R
.
I
FSET
R
V V
MAX
I
12
13
FB
DS(ON)
IMAX
–
U
GS
EXT
1k
TG
BG
from activating the current
applied to the MOSFET is
DS(ON)
•
0.1 F
CC
10
W
1
. To ensure proper
V
kHz
IN
A
Q1
Q2
varies as much as
CC1
CC1
L
CC1
O
can go low
must be at
is ramping
U
+
+
38311 F02
C
C
IN
V
OUT
MAX
OUT
where R
nected between FREQSET and the external voltage source
V
forces 15 A out of the pin, causing the internal oscillator
to run at approximately 450kHz. Forcing an external 20 A
current into FREQSET cuts the internal frequency to 100kHz.
An internal clamp prevents the oscillator from running
slower than about 50kHz. Tying FREQSET to V
the chip to run at this minimum speed.
Shutdown
The LTC3831-1 includes a low power shutdown mode,
controlled by the logic at the SHDN pin. A high at SHDN
allows the part to operate normally. A low level at SHDN for
more than 100 s forces the LTC3831-1 into shutdown
mode. In this mode, all internal switching stops, the COMP
and SS pins pull to ground and Q1 and Q2 turn off. The
LTC3831-1 supply current drops to <10 A, although off-
state leakage in the external MOSFETs may cause the total
V
temperatures. If SHDN returns high, the LTC3831-1 re-
runs a soft-start cycle and resumes normal operation.
External Clock Synchronization
The LTC3831-1 SHDN pin doubles as an external clock
input for applications that require a synchronized clock.
An internal circuit forces the LTC3831-1 into external
synchronization mode if a negative transition at the SHDN
pin is detected. In this mode, every negative transition on
the SHDN pin resets the internal oscillator and pulls the
ramp signal low. This forces the LTC3831-1 internal
oscillator to lock to the external clock frequency.
The LTC3831-1 internal oscillator can be externally syn-
chronized from 100kHz to 500kHz. Frequencies above
300kHz can cause a decrease in the maximum obtainable
duty cycle as rise/fall time and propagation delay take up
a larger percentage of the switch cycle. The low period of
this clock signal must not be >100 s or else the LTC3831-1
enters into the shutdown mode.
Figure 3 describes the operation of the external synchro-
nization function. A negative transition at the SHDN pin
forces the internal ramp signal low to restart a new PWM
cycle. Notice that the ramp amplitude is lowered as the
EXT
IN
current to be somewhat higher, especially at elevated
. Connecting an 82k resistor from FREQSET to ground
FSET
is a frequency programming resistor con-
CC
forces
38311f