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LTC4090/LTC4090-5

4090fa

USB and 5V Wall Adapter Power

Although the LTC4090/LTC4090-5 are 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/LTC4090-5 when 5V wall power is

not available. When 5V wall power is available, diode D1

supplies power to the LTC4090/LTC4090-5, 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 efﬁciency, 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 efﬁciency, lower maximum input

voltage, and higher dropout voltage. The highest acceptable

switching frequency (fSW(MAX)) for a given application can

be calculated as follows:

SW MAX

HVOUT

ON MIN

HVIN

()

() •–

()

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 ﬁnite 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

MN1

2.87k

59k

PROG

CLPROG

LTC4090

BAT

MP1

ICHG

Li-Ion

BATTERY

5V WALL

ADAPTER

850mA ICHG

USB POWER

500mA ICHG

4090 F04

LTC4090 Datasheet(PDF) 18 Page - Linear Technology

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