10
LTC3830/LTC3830-1
3830fa
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
I
IMAX
and I
FB
pins must be Kelvin sensed at Q1’s drain and
source pins. In addition, connect a 0.1µF decoupling
capacitor across R
IMAX
to filter switching noise. Other-
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
DS(ON)
, the actual current limit trip point is not highly
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
DS(ON)
of Q1 varies. Typically, R
DS(ON)
varies as much as
±40% and with ±25% variation on the LTC3830’s I
MAX
current, this can give a ±65% variation on the current limit
threshold.
The R
DS(ON)
is high if the V
GS
applied to the MOSFET is
low. This occurs during power up, when PV
CC1
is ramping
up. To prevent the high R
DS(ON)
from activating the current
limit, the LTC3830 disables the current limit circuit if
PV
CC1
is less than 2.5V above V
CC
. To ensure proper
operation of the current limit circuit, PV
CC1
must be at
least 2.5V above V
CC
when G1 is high. PV
CC1
can go low
when G1 is low, allowing the use of an external charge
pump to power PV
CC1
.
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 1µA 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 25µA out
of the pin, causing the internal oscillator to run at approxi-
mately 450kHz. Forcing an external 10µA 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
CC
forces the chip to run
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 100µs 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 <10µA, although off-
state leakage in the external MOSFETs may cause the total
V
IN
current to be some what higher, especially at elevated
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.
APPLICATIO S I FOR ATIO
WUUU
+
+
12
13
LTC3830
CC
12µA
0.1µF
Q2
C
OUT
3830 F04
C
IN
V
IN
V
OUT
G2
I
MAX
R
IMAX
I
FB
1k
+
–
Q1
L
O
G1
Figure 4. Current Limit Setting
