LTC3850EUF#PBF Linear Technology, LTC3850EUF#PBF Datasheet - Page 15

• Manufacturer Part Number: LTC3850EUF#PBF
• Description: IC CNTRLR STP DWN SYNC 28-QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3850EUF#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 23.3 V
• Current - Output: 100mA
• Frequency - Switching: 250kHz ~ 780kHz
• Voltage - Input: 4 ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-QFN
• Primary Input Voltage: 24V
• No. Of Outputs: 2
• Output Current: 100mA
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Supply Voltage Range: 4V To 24V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3850 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,
conduction loss through a sense resistor would cost sev-
eral 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’ datasheets for detailed information.
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.
500ns/DIV
500ns/DIV
F
= 1000pF , R
38501 F03
38501 F04
F
= 100Ω.
V
ESL(STEP)
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value
is:
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 (20mV, 40mV, or 60mV, depending on the state of
the I
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 coefficient 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:
R
R
R1||R2 =
R1=
P
LOSS
LIM
D
SENSE(EQUIV)
=
pin).
R1|| R2
DCR
R1=
R
R
D
(DCR at 20°C) • C1
SENSE(MAX)
(MAX)
SENSE(EQUIV)
(
V
LTC3850/LTC3850-1
; R2 =
IN(MAX)
=
at T
I
V
(MAX)
SENSE(MAX)
L
L(MAX)
) in the Electrical Characteristics
− V
R1 • R
1− R
R1
+
OUT
∆I
2
D
L
D
)
• V
OUT
L(MAX)
is 100°C.
15
38501fc