LTC3899_15 Datasheet, PDF (20/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Applications Information
BOOST
TG
SW
LTC3899
BG
R_SENSE
V_IN1,2
(V_OUT3)
V_OUT1,2
(V_IN3)
SENSE1,2⁺
(SENSE3^–)
SENSE1,2^–
(SENSE3⁺)
GND
CAP
PLACED NEAR SENSE PINS
3899 F04a
(2a) Using a Resistor to Sense Current
V_IN1,2
(V_OUT3)
BOOST
TG
SW
LTC3899
BG
INDUCTOR
L DCR
V_OUT1,2
(V_IN3)
SENSE1,2⁺
(SENSE3^–)
SENSE1,2^–
(SENSE3⁺)
R1
C1* _R2
GND
*PLACE C1 NEAR SENSE PINS
(R1||R2) • C1 = L/DCR
R_SENSE(EQ) = DCR(R2/(R1+R2))
3899 F04b
(2b) Using the Inductor DCR to Sense Current
Figure 2. Current Sensing Methods
duty factor. A curve is provided in the Typical Performance
Characteristics section to estimate this reduction in peak
inductor current depending upon the operating duty factor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3899 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 2b. 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, power loss through a sense resistor would
cost several points of efficiency compared to inductor
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
+
∆I_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 minimum value
for the maximum current sense threshold (V_SENSE(MAX))
for the LTC3899 is 65mV.
Next, determine the DCR of the inductor. When provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of copper 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 (R_D), use the divider ratio:
R_D
=
R_{SENSE(EQUIV )}
DCR_MAX at T_L(MAX)
3899f
20
For more information www.linear.com/LTC3899