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參數(shù)資料
| 型號： | LTC4090EDJC#TRPBR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO22 |
| 封裝： | 6 X 3 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-229WXXX, DFN-22 |
| 文件頁數(shù)： | 10/28頁 |
| 文件大小： | 392K |
| 代理商： | LTC4090EDJC#TRPBR |
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LTC4090
18
4090f
USB and 5V Wall Adapter Power
Although the LTC4090 is designed to draw power from
a USB port, a higher power 5V wall adapter can also be
used to power the application and charge the battery
(higher voltage wall adapters can be connected directly to
HVIN). Figure 4 shows an example of combining a 5V wall
adapter and a USB power input. With its gate grounded
by 1k, P-channel MOSFET MP1 provides USB power to
the LTC4090 when 5V wall power is not available. When
5V wall power is available, diode D1 supplies power to
the LTC4090, pulls the gate of MN1 high to increase the
charge current (by increasing the input current limit),
and pulls the gate of MP1 high to disable it and prevent
conduction back to the USB port.
Setting the Switching Frequency
The high voltage switching regulator uses a constant
frequency PWM architecture that can be programmed to
switch from 200kHz to 2.4MHz by using a resistor tied
from the RT pin to ground. A table showing the necessary
RT value for a desired switching frequency is in Table 1.
APPLICATIONS INFORMATION
Figure 4. USB or 5V Wall Adapter Power
Operating Frequency Tradeoffs
Selection of the operating frequency for the high voltage
buck regulator is a tradeoff between efciency, component
size, minimum dropout voltage, and maximum input volt-
age. The advantage of high frequency operation is that
smaller inductor and capacitor values may be used. The
disadvantages are lower efciency, lower maximum input
voltage, and higher dropout voltage. The highest acceptable
switching frequency (fSW(MAX)) for a given application can
be calculated as follows:
f
VV
tV
V
SW MAX
D
HVOUT
ON MIN
D
HVIN
SW
()
() –
=
+
()
where VHVIN is the typical high voltage input voltage,
VHVOUT is the output voltage of the switching regulator, VD
is the catch diode drop (~0.5V), and VSW is the internal
switch drop (~0.5V at max load). This equation shows
that slower switching frequency is necessary to safely
accommodate high VHVIN/VHVOUT ratio. Also, as shown in
the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the high voltage switch
has nite minimum on and off times. The switch can turn
on for a minimum of ~150ns and turn off for a minimum
of ~150ns. This means that the minimum and maximum
duty cycles are:
DCMIN = fSW tON(MIN)
DCMAX = 1 – fSW tOFF(MIN)
where fSW is the switching frequency, tON(MIN) is the
minimum switch-on time (~150ns), and tOFF(MIN) is the
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
0.2
187
0.3
121
0.4
88.7
0.5
68.1
0.6
56.2
0.7
46.4
0.8
40.2
0.9
34.0
1.0
29.4
1.2
23.7
1.4
19.1
1.6
16.2
1.8
13.3
2.0
11.5
2.2
9.76
2.4
8.66
2k
1k
MN1
2.87k
59k
IN
PROG
CLPROG
LTC4090
BAT
+
MP1
D1
ICHG
Li-Ion
BATTERY
5V WALL
ADAPTER
850mA ICHG
USB POWER
500mA ICHG
4090 F04
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