Datasheet

10
LTC3832/LTC3832-1
sn3832 3832fs
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
=
()()
()
()( )
VV V
fLV
IN OUT OUT
OSC O IN
–
f
OSC
= LTC3832 oscillator frequency = 300kHz
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
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 maximum current to be greater than the desired
current limit set point. Due to switching noise and varia-
tion 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 ±33% variation on the
LTC3832’s I
MAX
current, this can give a ±73% 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 LTC3832 will disable 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 the external charge
pump to power PV
CC1
.
APPLICATIO S I FOR ATIO
WUUU
Figure 4. Current Limit Setting
+
+
12
13
LTC3832
CC
12µA
0.1µF
Q2
C
OUT
3832 F04
C
IN
V
IN
V
OUT
G2
I
MAX
R
IMAX
I
FB
1k
+
–
Q1
L
O
G1
Oscillator Frequency
The LTC3832 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 1µA of current into/out
of the FREQSET pin decreases/increases the frequency by
10kHz. The pin is internally servoed to 1.265V. The
frequency can be estimated as:
f kHz
VV
R
kHz
A
EXT
FSET
=+
µ
300
1 265 10
1
.–
•
where R
FSET
is a frequency programming resistor con-
nected between FREQSET and the external voltage source
V
EXT
.