LTC3859_15 Datasheet, PDF (21/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
APPLICATIONS INFORMATION
INTV_CC
V_IN1,2
(V_OUT3)
BOOST
TG
LTC3859
SW
BG
R_SENSE
V_OUT1,2
(V_IN3)
SENSE1,2⁺
(SENSE3^–)
SENSE1, 2^–
(SENSE3⁺)
SGND
3859 F04a
CAP
PLACED NEAR SENSE PINS
4a. Using a Resistor to Sense Current
INTV_CC
V_IN1,2
(V_OUT3)
BOOST
TG
LTC3859
SW
BG
INDUCTOR
L DCR
SENSE1, 2⁺
(SENSE3^–)
SENSE1, 2^–
(SENSE3⁺)
R1
C1* _R2
SGND
3859 F04b
*PLACE C1 NEAR SENSE PINS
(R1||R2) • C1 = L/DCR
R_SENSE(EQ) = DCR(R2/(R1+R2))
4b. Using the Inductor DCR to Sense Current
Figure 4. Current Sensing Methods
V_OUT1,2
(V_IN3)
Inductor DCR Sensing
For applications requiring the highest possible efficiency at
high load currents, the LTC3859 is capable of sensing the
voltage drop across the inductor DCR, as shown in Figure 4b.
The DCR of the inductor represents the small amount of
DC winding resistance of the copper, which can be less
than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost several
points of efficiency compared to DCR sensing.
If the external R1||R2 • C1 time constant is chosen to be
exactly equal to the L/DCR time constant, the voltage drop
across the external capacitor is equal to the drop across
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
always the same and varies with temperature; consult the
manufacturers’ data sheets for detailed information.
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value
is:
R_{SENSE(EQUIV )}
=
V_{SENSE(MAX )}
I_MAX
+
ΔI_L
2
To ensure that the application will deliver full load cur-
rent over the full operating temperature range, determine
R_SENSE(EQUIV), keeping in mind that the maximum current
sense threshold (V_SENSE(MAX)) for the LTC3859 is fixed
at 50mV.
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of resistance, which is approximately 0.4%/°C.
A conservative value for T_L(MAX) is 100°C.
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
R_D
=
R_{SENSE(EQUIV )}
DCR_MAX at T_L(MAX)
C1 is usually selected to be in the range of 0.1μF to 0.47μF.
This forces R1||R2 to around 2k, reducing error that might
have been caused by the SENSE⁺ pin’s ±1μA current.
The equivalent resistance R1||R2 is scaled to the room
temperature inductance and maximum DCR:
R1(R2 =
L
(DCR at 20°C) • C1
The sense resistor values are:
R1= ^R1(R2 ; R2 = ^{R1• RD}
R_D
1− R_D
3859fa
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