ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 14

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

LTC3850-2
APPLICATIONS INFORMATION
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 fi lter 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’ datasheets for detailed information.
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value is:
14
R
SENSE(EQUIV)
20mV/DIV
20mV/DIV
Figure 4. Voltage Waveform Measured After the
Sense Resistor Filter. C
V
V
SENSE
SENSE
Figure 3. Voltage Waveform Measured
Directly Across the Sense Resistor.
=
V
I
(MAX)
SENSE(MAX)
500ns/DIV
500ns/DIV
+
F
ΔI
= 1000pF , R
2
L
38502 F03
38502 F04
F
= 100Ω.
V
ESL(STEP)
To ensure that the application will deliver full load cur-
rent over the full operating temperature range, choose
the minimum value for the Maximum Current Sense
Threshold (V
table (40mV).
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 tempera-
ture coeffi cient of resistance, which is approximately
0.4%/°C. A conservative value for T
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
C1 is usually selected to be in the range of 0.047μF to
0.47μF . This forces R1|| R2 to around 2kΩ, reducing
error that might have been caused by the SENSE pins’
±1μA current.
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
Ensure that R1 has a power rating higher than this value.
If high effi ciency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
R
R1||R2 =
R1=
P
LOSS
D
=
R1|| R2
DCR
R1=
R
R
D
(DCR at 20°C) • C1
SENSE(MAX)
(MAX)
SENSE(EQUIV)
(
V
; R2 =
IN(MAX)
at T
L
L(MAX)
) in the Electrical Characteristics
− V
R1 • R
1− R
R1
OUT
D
D
)
• V
OUT
L(MAX)
is 100°C.
38502f