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| 型号： | LTC3708EUH#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/32页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 32-QFN |
| 标准包装： | 2,500 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 占空比： | 90% |
| 电源电压： | 4 V ~ 36 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： | 32-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
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LTC3708
APPLICATIONS INFORMATION
The basic LTC3708 application circuit is shown on the ?rst
page of this data sheet. External component selection is
primarily determined by the maximum load current and
begins with the selection of the power MOSFET switches
and/or sense resistor. For the LTC3708, the inductor cur-
rent is determined by the R DS(ON) of the synchronous
MOSFET or by a sense resistor when the user opts for
more accurate current sensing. The desired amount of
ripple current and operating frequency largely determines
the inductor value. Finally, C IN is selected for its ability to
handle the large RMS current into the converter and C OUT
is chosen with low enough ESR to meet the output voltage
ripple speci?cation.
Maximum Sense Voltage and V RNG Pin
Inductor current is determined by measuring the voltage
across the R DS(ON) of the synchronous MOSFET or through
a sense resistor that appears between the SENSE + and
SENSE – pins. The maximum sense voltage is set by the
voltage applied to the V RNG pin and is equal to approximately
V RNG /7. The current mode control loop will not allow the
inductor current valleys to exceed V RNG /(7 ? R SENSE ). In
practice, one should allow some margin for variations in
the LTC3708 and external component values. A good guide
for selecting the sense resistance is:
across this resistor, connect the SENSE + pin to the source
of the synchronous MOSFET and the SENSE – pin to the
other end of the resistor. The SENSE + and SENSE – pins
provide the Kelvin connections, ensuring accurate voltage
measurement across the resistor. Using a sense resistor
provides a well-de?ned current limit, but adds cost and
reduces ef?ciency. Alternatively, one can use the synchro-
nous MOSFET as the current sense element by simply
connecting the SENSE + pin to the switch node SW and
the SENSE – pin to the source of the synchronous MOSFET,
eliminating the sense resistor. This improves ef?ciency,
but one must carefully choose the MOSFET on-resistance
as discussed below.
Power MOSFET Selection
Each output stage of the LTC3708 requires two external
N-channel power MOSFETs, one for the top (main) switch
and one for the bottom (synchronous) switch. Important
parameters for the power MOSFETs are the breakdown
voltage V (BR)DSS , threshold voltage V GS(TH) , on-resistance
R DS(ON) , reverse transfer capacitance, C RSS , and maximum
current, I DS(MAX) .
The gate drive voltage is set by the 5V DRV CC supply.
Consequently, logic-level threshold MOSFETs must be
used in LTC3708 applications. If the driver’s voltage is
R SENSE =
V RNG
10 ? I OUT ( MAX )
expected to drop below 5V, then sub-logic level threshold
MOSFETs should be considered.
R DS ( ON )( MAX ) =
The voltage of the V RNG pin can be set using an external
resistive divider from V CC between 0.5V and 2V, resulting
in nominal sense voltages of 50mV to 200mV. Additionally,
the V RNG pin can be tied to ground or V CC , in which case
the nominal sense voltage defaults to 70mV or 140mV,
respectively. The maximum allowed sense voltage is about
1.4 times this nominal value.
Connecting the SENSE + and SENSE – Pins
The LTC3708 provides the user with an optional method
to sense current through a sense resistor instead of using
the R DS(ON) of the synchronous MOSFET. When using a
sense resistor, it is placed between the source of the syn-
chronous MOSFET and ground. To measure the voltage
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-re-
sistance. MOSFET on-resistance is typically speci?ed
with a maximum value R DS(ON)(MAX) at 25°C. Additional
margin is required to accommodate the rise in MOSFET
on-resistance with temperature:
R SENSE
ρ T
The ρ T term is a normalization factor (unity at 25°C) ac-
counting for the signi?cant variation in on-resistance with
temperature, typically about 0.4%/°C. For a maximum
junction temperature of 100°C, using a value ρ 100°C =
1.3 is reasonable (see Figure 3).
3708fb
13
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相关代理商/技术参数 |
参数描述 |
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| LTC3709EG#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 36-SSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LTC3709EG#TRPBF | 功能描述:IC REG CTRLR BUCK PWM CM 36-SSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LTC3709EUH | 制造商:Linear Technology 功能描述:DC DC Cntrlr Single-OUT Sync Step Down 5V to 36V Input 32-Pin QFN EP |
| LTC3709EUH#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 32-QFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LTC3709EUH#TRPBF | 功能描述:IC REG CTRLR BUCK PWM CM 32-QFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
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