LTC3850EUF#PBF Linear Technology, LTC3850EUF#PBF Datasheet - Page 13

• 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
The Typical Application on the first page is a basic LTC3850
application circuit. LTC3850 can be configured to use either
DCR (inductor resistance) sensing or low value resistor
sensing. The choice between the two current sensing
schemes is largely a design trade-off between cost, power
consumption, and accuracy. DCR sensing is becoming
popular because it saves expensive current sensing resis-
tors and is more power efficient, especially in high current
applications. However, current sensing resistors provide
the most accurate current limits for the controller. Other
external component selection is driven by the load require-
ment, and begins with the selection of R
used) and inductor value. Next, the power MOSFETs are se-
lected. Finally, input and output capacitors are selected.
Current Limit Programming
The I
mum current limit of the controller. When I
grounded, floated or tied to INTV
the maximum current sense threshold will be 30mV, 50mV
or 75mV, respectively.
Which setting should be used? For the best current limit
accuracy, use the 75mV setting. The 30mV setting will allow
for the use of very low DCR inductors or sense resistors,
but at the expense of current limit accuracy. The 50mV
setting is a good balance between the two. For single output
dual phase applications (see Figure 21), use the 50mV or
75mV setting for optimal current sharing.
LIM
38501 F02a
pin is a tri-level logic input which sets the maxi-
LTC3850
SGND
(2a) Using a Resistor to Sense Current
SENSE
SENSE
INTV
BOOST
PGND
SW
V
BG
TG
CC
PLACED NEAR SENSE PINS
IN
+
–
FILTER COMPONENTS
C
F
R
R
F
F
CC
, the typical value for
Figure 2. Two Different Methods of Sensing Current
V
SENSE RESISTOR
C
PLUS PARASITIC
CANCELLATION
IN
F
INDUCTANCE
R
• 2
POLE-ZERO
S
SENSE
RF
≤ ESL/R
ESL
LIM
(if R
S
V
OUT
is either
SENSE
is
SENSE
The SENSE
comparators. The common mode input voltage range of
the current comparators is 0V to 5V. Both SENSE pins are
high impedance inputs with small base currents of less
than 1µA. When the SENSE pins ramp up from 0V to 1.4V,
the small base currents flow out of the SENSE pins. When
the SENSE pins ramp down from 5V to 1.1V, the small base
currents flow into the SENSE pins. The high impedance
inputs to the current comparators allow accurate DCR
sensing. However, care must be taken not to float these
pins during normal operation.
Filter components mutual to the sense lines should be
placed close to the LTC3850, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 1). Sensing cur-
rent elsewhere can effectively add parasitic inductance
and capacitance to the current sense element, degrading
the information at the sense terminals and making the
programmed current limit unpredictable. If DCR sensing
is used (Figure 2b), sense resistor R1 should be placed
*PLACE C1 NEAR SENSE
38501 F02b
SENSE
+
LTC3850
–
(2b) Using the Inductor DCR to Sense Current
SGND
and SENSE
PINS
+
SENSE
SENSE
INTV
BOOST
PGND
and SENSE
Figure 1. Sense Lines Placement
with Inductor or Sense Resistor
V
SW
TG
BG
CC
IN
+
–
LTC3850/LTC3850-1
+
,
INDUCTOR OR R
C1*
–
Pins
R1 || R2
–
TO SENSE FILTER,
NEXT TO THE CONTROLLER
pins are the inputs to the current
× C1 =
R2
SENSE
DCR
R1
L
C
OUT
R
SENSE(EQ)
38501 F01
INDUCTOR
L
V
= DCR
IN
DCR
R1 + R2
R2
V
13
OUT
38501fc