LTC3892-1_15 Datasheet, PDF (17/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Applications Information
BOOST
LTC3892/ _TG
LTC3892-1
SW
BG
R_SENSE
V_IN1,2
V_OUT1,2
SENSE1,2⁺
SENSE1,2^–
CAP
PLACED NEAR SENSE PINS
GND
38921 F04a
(2a) Using a Resistor to Sense Current
V_IN1,2
BOOST
LTC3892/
LTC3892-1
TG
SW
BG
INDUCTOR
L DCR
V_OUT1,2
SENSE1,2⁺
SENSE1,2^–
R1
C1* _R2
GND
*PLACE C1 NEAR SENSE PINS
(R1||R2) • C1 = L/DCR
R_SENSE(EQ) = DCR(R2/(R1+R2))
38921 F04b
(2b) Using the Inductor DCR to Sense Current
Figure 2. Current Sensing Methods
would cost several points of efficiency compared to induc-
tor 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 current
over the full operating temperature range, choose the
minimum value for V_SENSE(MAX) in the Electrical Charac-
teristics table.
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)
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

=
(DCR
at
L
20°C)
•
C1

The sense resistor values are:
R1=
R1 R2

R_D
;
R2
=
R1•R_D
1− R_D

The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
( ) _{PLOSS R1=}
V_IN(MAX) − V_OUT
R1
• V_OUT
For more information www.linear.com/LTC3892
38921f
17