LTC3830-1 Linear Technology, LTC3830-1 Datasheet - Page 10

• Manufacturer Part Number: LTC3830-1
• Description: High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation
• Manufacturer: Linear Technology

LTC3830/LTC3830-1
APPLICATIO S I FOR ATIO
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
I
source pins. In addition, connect a 0.1 F decoupling
capacitor across R
wise, noise spikes or ringing at Q1’s source can cause the
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 LTC3830 disables the current limit circuit if
PV
operation of the current limit circuit, PV
least 2.5V above V
when G1 is low, allowing the use of an external charge
pump to power PV
10
IMAX
LTC3830
40% and with 25% variation on the LTC3830’s I
DS(ON)
DS(ON)
CC1
and I
DS(ON)
DS(ON)
CC
is less than 2.5V above V
, the actual current limit trip point is not highly
of Q1 varies. Typically, R
+
–
FB
of Q1 usually increases with temperature. To
pins must be Kelvin sensed at Q1’s drain and
is high if the V
Figure 4. Current Limit Setting
12 A
CC
CC1
IMAX
U
when G1 is high. PV
.
I
R
MAX
MAX
I
12
13
FB
IMAX
DS(ON)
to filter switching noise. Other-
U
GS
is designed with a positive
1k
G1
G2
from activating the current
applied to the MOSFET is
DS(ON)
0.1 F
CC
W
DS(ON)Q1
V
. To ensure proper
IN
Q1
Q2
varies as much as
CC1
CC1
L
CC1
O
.
must be at
can go low
is ramping
U
+
+
3830 F04
C
C
IN
OUT
V
MAX
OUT
Oscillator Frequency
The LTC3830 includes an onboard current controlled
oscillator that typically free-runs at 200kHz. 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 200kHz. 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, connecting
a 50k resistor from FREQSET to ground forces 25 A out
of the pin, causing the internal oscillator to run at approxi-
mately 450kHz. Forcing an external 10 A current into
FREQSET cuts the internal frequency to 100kHz. An inter-
nal clamp prevents the oscillator from running slower than
about 50kHz. Tying FREQSET to V
at this minimum speed. The LTC3830-1 and the 8-lead
LTC3830 do not have this frequency adjustment function.
Shutdown
The LTC3830 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 LTC3830 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
LTC3830 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 LTC3830 reruns
a soft-start cycle and resumes normal operation. The
LTC3830-1 does not have this shutdown function.
External Clock Synchronization
The LTC3830 SHDN pin doubles as an external clock input
for applications that require a synchronized clock. An
internal circuit forces the LTC3830 into external synchro-
nization 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 LTC3830 internal oscillator to
lock to the external clock frequency. The LTC3830-1 does
not have this external synchronization function.
IN
current to be some what higher, especially at elevated
CC
forces the chip to run
sn3830 3830fs