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| 型号： | LTC3812IFE-5#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/34页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16TSSOP |
| 标准包装： | 95 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 93% |
| 电源电压： | 6.2 V ~ 60 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： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 管件 |
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LTC3812-5
APPLICATIONS INFORMATION
The basic LTC3812-5 application circuit is shown on the
?rst page of this data sheet. External component selection
is primarily determined by the maximum input voltage and
load current and begins with the selection of the power
MOSFET switches. The LTC3812-5 uses the on-resistance
of the synchronous power MOSFET for determining the
inductor current. The desired amount of ripple current
and operating frequency largely determines the inductor
value. Next, 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
and transient speci?cation. Finally, loop compensation
components are selected to meet the required transient/
phase margin speci?cations.
POWER MOSFET SELECTION
The LTC3812-5 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 BV DSS ,
threshold voltage V (GS)TH , on-resistance R DS(ON) , input
capacitance and maximum current I DS(MAX) .
Since the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-resis-
tance. MOSFET on-resistance is typically speci?ed with
a maximum value R DS(ON)(MAX) at 25°C. In this case,
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
MAXIMUM SENSE VOLTAGE AND V RNG PIN
R DS(ON)(MAX) =
R SENSE
T
Inductor current is determined by measuring the voltage
across a sense resistance (the on-resistance of the bottom
MOSFET) that appears between the PGND and SW pins.
The maximum sense voltage is set by the voltage applied
to the V RNG pin and is equal to approximately:
V SENSE(MAX) = 0.173V RNG – 0.026
The current mode control loop will not allow the inductor
current valleys to exceed V SENSE(MAX) /R SENSE . In prac-
tice, one should allow some margin for variations in the
LTC3812-5 and external component values and a good
guide for selecting the sense resistance is:
The ρ T term is a normalization factor (unity at 25°C)
accounting for the signi?cant variation in on-resistance
with temperature (see Figure 5) and typically varies
from 0.4%/°C to 1.0%/°C depending on the particular
MOSFET used.
2.0
1.5
R SENSE =
V SENSE(MAX)
1.3 ?I OUT(MAX)
1.0
An external resistive divider from INTV CC can be used
to set the voltage of the V RNG pin between 0.5V and 2V
0.5
resulting in nominal sense voltages of 60mV to 320mV.
Additionally, the V RNG pin can be tied to SGND or INTV CC
in which case the nominal sense voltage defaults to 95mV
0
–50
0 50 100
JUNCTION TEMPERATURE ( ° C)
150
38125 F05
or 215mV, respectively.
Figure 5. R DS(ON) vs Temperature
38125fc
14
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