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參數(shù)資料
| 型號(hào)： | LTC3812IFE-5 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 6/32頁 |
| 文件大小： | 425K |
| 代理商： | LTC3812IFE-5 |
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LTC3812-5
14
38125fb
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, CIN is selected for its ability to handle the
large RMS current into the converter and COUT is chosen
with low enough ESR to meet the output voltage ripple
and transient specication. Finally, loop compensation
components are selected to meet the required transient/
phase margin specications.
MAXIMUM SENSE VOLTAGE AND VRNG PIN
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 VRNG pin and is equal to approximately:
VSENSE(MAX) = 0.173VRNG – 0.026
The current mode control loop will not allow the inductor
current valleys to exceed VSENSE(MAX)/RSENSE. 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:
RSENSE =
VSENSE(MAX)
1.3 IOUT(MAX)
An external resistive divider from INTVCC can be used
to set the voltage of the VRNG pin between 0.5V and 2V
resulting in nominal sense voltages of 60mV to 320mV.
Additionally, the VRNG pin can be tied to SGND or INTVCC
in which case the nominal sense voltage defaults to 95mV
or 215mV, respectively.
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 BVDSS,
threshold voltage V(GS)TH, on-resistance RDS(ON), input
capacitance and maximum current IDS(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 specied with
a maximum value RDS(ON)(MAX) at 25°C. In this case,
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
RDS(ON)(MAX) =
RSENSE
T
The ρT term is a normalization factor (unity at 25°C)
accounting for the signicant 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.
Figure 5. RDS(ON) vs Temperature
APPLICATIONS INFORMATION
JUNCTION TEMPERATURE (°C)
–50
ρ
TNORMALIZED
ON-RESISTANCE
1.0
1.5
150
38125 F05
0.5
0
50
100
2.0
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