Datasheet

LTC4090/LTC4090-5
18
4090fc
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 R
T
pin to ground. A table showing the necessary
R
T
value for a desired switching frequency is in Table 1.
Table 1. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz)
R
T
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
APPLICATIONS INFORMATION
Figure 4. USB or 5V Wall Adapter Power
Operating Frequency Trade-Offs
Selection of the operating frequency for the high voltage
buck regulator is a trade-off between effi ciency, compo-
nent size, minimum dropout voltage, and maximum input
voltage. The advantage of high frequency operation is that
smaller inductor and capacitor values may be used. The
disadvantages are lower effi ciency, lower maximum input
voltage, and higher dropout voltage. The highest acceptable
switching frequency (f
SW(MAX)
) for a given application can
be calculated as follows:
f
SW(MAX)
=
V
D
+ V
HVOUT
t
ON(MIN)
•V
D
+ V
HVIN
–V
SW
()
where V
HVIN
is the typical high voltage input voltage,
V
HVOUT
is the output voltage of the switching regulator, V
D
is the catch diode drop (~0.5V), and V
SW
is the internal
switch drop (~0.5V at max load). This equation shows
that slower switching frequency is necessary to safely
accommodate high V
HVIN
/V
HVOUT
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 fi 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:
DC
MIN
= f
SW
• t
ON(MIN)
DC
MAX
= 1 – f
SW
• t
OFF(MIN)
where f
SW
is the switching frequency, t
ON(MIN)
is the
minimum switch-on time (~150ns), and t
OFF(MIN)
is the
2k1k
MN1
2.87k
59k
IN
PROG
CLPROG
LTC4090
BAT
+
MP1
D1
I
CHG
Li-Ion
BATTERY
5V WALL
ADAPTER
850mA I
CHG
USB POWER
500mA I
CHG
4090 F04