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參數(shù)資料
| 型號： | LTC3772ETS8#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 1 MM HEIGHT, PLASTIC, MO-193, SOT-23, 8 PIN |
| 文件頁數(shù)： | 2/20頁 |
| 文件大小： | 248K |
| 代理商： | LTC3772ETS8#PBF |
10
LTC3772
3772f
The basic LTC3772 application circuit is shown on the front
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of the power MOSFET inductor and the output diode.
These are selected followed by the input bypass capacitor
CIN and output bypass capacitor COUT.
Power MOSFET Selection
An external P-channel power MOSFET must be selected for
use with the LTC3772. The main selection criteria for the
power MOSFET are the threshold voltage VGS(TH) and the
“on” resistance RDS(ON), reverse transfer capacitance CRSS
and total gate charge.
Since the LTC3772 is designed for operation down to low
input voltages, a sublogic level threshold 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 LTC3772 is less than
the absolute maximum VGS rating.
The P-channel MOSFET’s on-resistance is chosen based on
the required load current. The maximum average output
load current IOUT(MAX) is equal to the peak inductor current
minus half the peak-to-peak ripple current IRIPPLE. The
LTC3772’s current comparator monitors the drain-to-
source voltage VDS of the 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 volt-
age on the ITH pin of the current comparator. The voltage
on the ITH pin is internally clamped, which limits the maxi-
mum current sense threshold VSENSE(MAX) to approxi-
mately 175mV when IPRG is floating (100mV when IPRG is
tied low; 250mV when IPRG is tied high).
The output current that the LTC3772 can provide is given by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
–
=
2
A reasonable starting point is setting ripple current IRIPPLE
to be 40% of IOUT(MAX). Rearranging the above equation
yields:
R
V
I
DS ON MAX
SENSE MAX
OUT MAX
()(
)
()
=
5
6
for Duty Cycle < 20%.
However, for operation above 20% duty cycle, slope com-
pensation has to be taken into consideration to select the
appropriate value of RDS(ON)toprovidetherequiredamount
of load current:
R
VSF
I
DS ON MAX
SENSE MAX
OUT MAX
()(
)
()
–
=
5
6
where SF is a 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 LTC3772
and external component values:
R
VSF
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 maximum
ambient temperature of 70°C, usingρ80°C~1.3intheabove
equation is a reasonable choice.
The required minimum RDS(ON) of the MOSFET is also
governed by its allowable power dissipation. For applica-
tions that may operate the LTC3772 in dropout–i.e., 100%
APPLICATIO S I FOR ATIO
WU
UU
Figure 2. RDS(ON) vs Temperature
JUNCTION TEMPERATURE (C)
–50
ρ
TNORMALIZED
ON
RESISTANCE
1.0
1.5
150
0.5
0
50
100
2.0
3772 F02
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