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| 型号： | LTC3856EUH#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/40页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 32WFQFN |
| 标准包装： | 73 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 850kHz |
| 占空比： | 94% |
| 电源电压： | 4.5 V ~ 38 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3856
APPLICATIONS INFORMATION
The Typical Application on the first page of this data sheet
is a basic LTC3856 application circuit. LTC3856 can be
configured to use either DCR (inductor resistance) sens-
ing 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 resistors 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 requirement, and begins with the se-
lection of R SENSE (if R SENSE is used) and inductor value.
Next, the power MOSFETs are selected. Finally, input and
output capacitors are selected.
Current Limit Programming
The I LIM pin is a tri-level logic input which sets the maxi-
mum current limit of the controller. When I LIM is either
grounded, floated or tied to INTV CC , the typical value for
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.
SENSE + and SENSE – Pins
The SENSE + and SENSE – pins are the inputs to the current
comparators. The common mode input voltage range of
the current comparators is 0V to 5V. All SENSE + pins are
high impedance inputs with small currents of less than
1μA. The positive high impedance input to the current
comparators allows accurate DCR sensing. All SENSE – pins
and DIFFP should be connected directly to V OUT when DCR
sensing is used. 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 LTC3856, and
the sense lines should run close together to a Kelvin con-
nection underneath the current sense element (shown in
Figure 1). Sensing current 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 close to the switching node, to prevent
noise from coupling into sensitive small-signal nodes. The
capacitor C1 should be placed close to the IC pins.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C OUT
R SENSE 3856 F01
Figure 1. Sense Lines Placement with Sense Resistor
V IN
INTV CC
V IN
V IN
INTV CC
LTC3856
V IN
BOOST
TG
SW
LTC3856
BG
PGND
SENSE +
SENSE –
SGND
C F
R F
R F
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R S ESL
C F ? 2R F ≤ ESL/R S
POLE-ZERO
CANCELLATION
V OUT
OPTIONAL
TEMP COMP
NETWORK
R S
R NTC R P
ITEMP
SGND
BOOST
TG
SW
BG
PGND
SENSE +
SENSE –
C1*
R2
R1
INDUCTOR
L DCR
V OUT
3856 F02a
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3856 F02b
*PLACE C1 NEAR SENSE + ,
SENSE – PINS
R1||R2 × C1 =
L
DCR
R SENSE(EQ) = DCR
R2
R1 + R2
(2a) Using a Resistor to Sense Current
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
3856f
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