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參數(shù)資料
| 型號： | LTC3809EMS#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 825 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | PLASTIC, MSOP-10 |
| 文件頁數(shù)： | 4/24頁 |
| 文件大?。?/td> | 266K |
| 代理商： | LTC3809EMS#PBF |
12
LTC3809
3809f
APPLICATIO S I FOR ATIO
WU
UU
The typical LTC3809 application circuit is shown in
Figure 10. External component selection for the controller
is driven by the load requirement and begins with the
selection of the inductor and the power MOSFETs.
Power MOSFET Selection
The LTC3809’s controller requires two external power
MOSFETs: a P-channel MOSFET for the topside (main)
switch and a N-channel MOSFET for the bottom (synchro-
nous) switch. The main selection criteria for the power
MOSFETs are the breakdown voltage VBR(DSS), threshold
voltage VGS(TH), on-resistance RDS(ON), reverse transfer
capacitance CRSS, turn-off delay tD(OFF) and the total gate
charge QG.
The gate drive voltage is the input supply voltage. Since the
LTC3809 is designed for operation down to low input
voltages, a sublogic level MOSFET (RDS(ON) guaranteed at
VGS = 2.5V) is required for applications that work close to
this voltage. When these MOSFETs are used, make sure
that the input supply to the LTC3809 is less than the
absolute maximum MOSFET VGS rating, which is typically
8V.
The P-channel MOSFET’s on-resistance is chosen based
on the required load current. The maximum average load
current IOUT(MAX) is equal to the peak inductor current
minus half the peak-to-peak ripple current IRIPPLE. The
LTC3809’s current comparator monitors the drain-to-
source voltage VDS of the top P-channel MOSFET, which
is sensed between the VIN and SW pins. The peak inductor
current is limited by the current threshold, set by the
voltage on the ITH pin, of the current comparator. The
voltage on the ITH pin is internally clamped, which limits
the maximum current sense threshold
VSENSE(MAX) to
approximately 125mV when IPRG is floating (85mV when
IPRG is tied low; 204mV when IPRG is tied high).
The output current that the LTC3809 can provide is given
by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
–
=
2
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point is setting ripple current IRIPPLE
to be 40% of IOUT(MAX). Rearranging the above equation
yields:
R
V
I
for Duty Cycle
DS ON MAX
SENSE MAX
OUT MAX
()
%
=
<
5
6
20
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of RDS(ON) to provide the required
amount of load current:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
=
5
6
where SF is a scale factor whose value is obtained from the
curve in Figure 1.
These must be further derated to take into account the
significant variation in on-resistance with temperature.
The following equation is a good guide for determining the
required RDS(ON)MAX at 25°C (manufacturer’s specifica-
tion), allowing some margin for variations in the LTC3809
and external component values:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
T
()
.
=
5
6
09
ρ
The
ρT is a normalizing term accounting for the tempera-
ture variation in on-resistance, which is typically about
0.4%/
°C, as shown in Figure 2. Junction-to-case tempera-
ture TJC is about 10°C in most applications. For a maxi-
mum ambient temperature of 70
°C, using ρ80°C ~ 1.3 in
the above equation is a reasonable choice.
The N-channel MOSFET’s on resistance is chosen based
on the short-circuit current limit (ISC). The LTC3809’s
short-circuit current limit comparator monitors the drain-
to-source voltage VDS of the bottom N-channel MOSFET,
which is sensed between the GND and SW pins. The
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