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| 型号： | LTC3876EFE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 23/48页 |
| 文件大小： | 0K |
| 描述： | IC CTLR DC/DC DDR DUAL 38-TSSOP |
| 产品培训模块： | LTC3876 Dual DC/DC Controller |
| 标准包装： | 50 |
| 应用： | 控制器，DDR，DDR2，DDR3 |
| 输入电压： | 4.5 V ~ 38 V |
| 输出数： | 2 |
| 输出电压： | 可调 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 38-TFSOP （0.173"，4.40mm 宽）裸露焊盘 |
| 供应商设备封装： | 38-TSSOP 裸露焊盘 |
| 包装： | 管件 |
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LTC3876
APPLICATIONS INFORMATION
the target sense resistance. With the ability to program
current limit through the V RNG pin, R2 may be optional. C1
is usually selected in the range of 0.01μF to 0.47μF. This
forces R1||R2 to around 2k to 4k, reducing error that might
have been caused by the SENSE pins’ input bias currents.
Resistor R1 should be placed close to the switching node,
to prevent noise from coupling into sensitive small-signal
nodes. Capacitor C1 should be placed close to the IC pins.
The first step in designing DCR current sensing is to
determine the DCR of the inductor. Where provided, use
the manufacturer’s maximum value, usually given at 25°C.
power loss when deciding whether to use DCR sensing or
R SENSE sensing. Light load power loss can be modestly
higher with a DCR network than with a sense resistor due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method.
To maintain a good signal-to-noise ratio for the current
sense signal, start with a ? V SENSE of 10mV. For a DCR
sensing application, the actual ripple voltage will be de-
termined by:
? OUT
Increase this value to account for the temperature coef-
ficient of resistance, which is approximately 0.4%/°C. A
conservative value for inductor temperature T L is 100°C.
Δ V SENSE =
V IN – V OUT
R1? C1
V
V IN ? f
? 1 + 0.4% ( T L(MAX) – 25 ° C ) ?
?
?
TheDCRoftheinductorcanalsobemeasuredusingagood
RLC meter, but the DCR tolerance is not always the same
and varies with temperature; consult the manufacturers’
data sheets for detailed information.
From the DCR value, V SENSE(MAX) is easily calculated as:
V SENSE(MAX) = DCR MAX(25 ° C)
?
Power MOSFET Selection
Two external N-channel power MOSFETs must be selected
for each channel of the LTC3876 controller: one for the
top (main) switch and one for the bottom (synchronous)
switch. The gate drive levels are set by the DRV CC voltage.
This voltage is typically 5.3V. Pay close attention to the
BV DSS specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
? ? I OUT(MAX) –
?
?
Δ I L ?
2 ? ?
Selection criteria for the power MOSFETs include the on-
resistance, R DS(ON) , Miller capacitance, C MILLER , input
voltage and maximum output current. Miller capacitance,
( V IN(MAX) – V OUT ) ? V
P LOSS ( ) =
D TOP =
D BOT = 1– OUT
IfV SENSE(MAX) iswithinthemaximumsensevoltage(30mV
to 100mV) of the LTC3876 as programmed by the V RNG
pin, then the RC filter only needs R1. If V SENSE(MAX) is
higher, then R2 may be used to scale down the maximum
sense voltage so that it falls within range.
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
OUT
R1
R1
Ensure that R1 has a power rating higher than this value.
C MILLER , can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C MILLER is equal to the increase in gate charge along
the horizontal axis while the curve is approximately flat,
divided by the specified V DS test voltage.
When the IC is operating in continuous mode, the duty
cycles for the top and bottom MOSFETs are given by:
V OUT
V IN
V
V IN
If high efficiency is necessary at light loads, consider this
3876f
23
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