LTC3832_15 Datasheet, PDF (10/24 Pages) Linear Technology – High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation

LTC3832/LTC3832-1
APPLICATIO S I FOR ATIO
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
( ) ⁼
(^V_IN ^{– V}_OUT)(^V_OUT
f_{OSC (}L_O)(V_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.
V_IN
LTC3832
+
CC
–
12µA
R_IMAX
0.1µF
12
I_MAX
I_FB
13
G1
1k
Q1
L_O
G2
Q2
Figure 4. Current Limit Setting
+
C_IN
+
V_OUT
C_OUT
3832 F04
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 = 300kHz + ^{1.265V – VEXT} • ^10kHz
R_FSET
1µA
where R_FSET is a frequency programming resistor con-
nected between FREQSET and the external voltage source
V_EXT.
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sn3832 3832fs