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| 型号： | LTC3850IGN-2#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 12/36页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-SSOP |
| 产品培训模块： | LTC3850 Dual Output DC/DC Switching Regulator Controller |
| 标准包装： | 2,500 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 频率 - 最大： | 860kHz |
| 占空比： | 97.2% |
| 电源电压： | 4 V ~ 30 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 ~ 85°C |
| 封装/外壳： | 28-SSOP（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
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LTC3850-2
APPLICATIONS INFORMATION
The Typical Application on the ?rst page is a basic LTC3850-
2 application circuit. LTC3850-2 can be con?gured to use
either DCR (inductor resistance) sensing or low value resis-
tor 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 ef?cient, 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 SENSE (if R SENSE is
used) and inductor value. Next, the power MOSFETs are se-
lected. Finally, input and output capacitors are selected.
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. 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 ?ow out of the SENSE
pins. When the SENSE pins ramp down from 5V to 1.1V,
the small base currents ?ow into the SENSE pins. The
Filter components mutual to the sense lines should be placed
close to the LTC3850-2, and the sense lines should run close
together to a Kelvin connection 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 unpre-
dictable. 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.
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R SENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
V SENSE(MAX) . The input common mode range of the current
comparator is 0V to 5V. The current comparator threshold
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C OUT
high impedance inputs to the current comparators allow
INDUCTOR OR R SENSE
38502 F01
accurate DCR sensing. However, care must be taken not
to ?oat these pins during normal operation.
Figure 1. Sense Lines Placement
with Inductor or Sense Resistor
V IN
INTV CC
V IN
V IN
INTV CC
V IN
BOOST
TG
SW
LTC3850-2
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R S ESL
V OUT
BOOST
TG
SW
LTC3850-2
INDUCTOR
L DCR
V OUT
BG
BG
PGND
SENSE +
SENSE –
C F
R F
C F ? 2 RF ≤ ESL/R S
POLE-ZERO
CANCELLATION
PGND
SENSE +
SENSE –
C1*
R2
R1
38502 F02a
SGND
R F
38502 F02b
SGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
*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
38502f
12
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