MAX849 MAXIM [Maxim Integrated Products], MAX849 Datasheet - Page 13

• Manufacturer Part Number: MAX849
• Description: 1-Cell to 3-Cell, High-Power, Low-Noise, Step-Up DC-DC Converters
• Manufacturer: MAXIM [Maxim Integrated Products]

Figure 9. Momentary Pushbutton On/Off Switch
A 22µF, low-ESR input capacitor will reduce peak cur-
rents and reflected noise due to inductor current ripple.
Smaller ceramic capacitors may also be used for light
loads or in applications that can tolerate higher input
ripple.
Two 100µF (single 100µF for the MAX848), 10V, low-
ESR, output filter capacitors typically exhibit 30mV rip-
ple when stepping up from 1.2V to 3.3V at 200mA
(100mA for the MAX848). Bypass the MAX848/MAX849
supply input, OUT, with a 0.1µF ceramic capacitor
to GND. Also bypass POUT to PGND with a 0.1µF
ceramic capacitor.
The filter capacitors’ equivalent series resistance (ESR)
affects efficiency and output ripple. The output voltage
ripple is the product of the peak inductor current and
the output capacitor’s ESR. Low-ESR, surface-mount
tantalum capacitors are currently available from
Sprague (595D series) and AVX (TPS series). Sanyo
OS-CON organic-semiconductor, through-hole capaci-
tors also exhibit very low ESR, and are especially useful
for operation at cold temperatures. See Table 5 for a list
of suggested capacitor suppliers.
Table 3. Available Output Power
1 NiCd/NiMH
2 NiCd/NiMH
2 NiCd/NiMH
3 NiCd/NiMH or 1 Li-Ion
NUMBER OF CELLS
1M
______________________________________________________________________________________
ON2
ON1
MAX848
MAX849
Low-Noise, Step-Up DC-DC Converters
1M
OUT
Input Bypass Capacitors
Capacitor Selection
Output Filter Capacitors
INPUT VOLTAGE
V
I/O
I/O
DD
(V)
1.2
2.4
2.4
3.6
C
1-Cell to 3-Cell, High-Power,
A momentary pushbutton switch can be used to turn
the MAX848/MAX849 on and off.
As shown in Figure 9, ON1 is pulled low and ON2 is
pulled high when the part is off. When the momentary
switch is pressed, ON2 is pulled low and the regulator
turns on. The switch should be on long enough for the
µC to exit reset. The controller issues a logic high to
ON1, which guarantees that the part will stay on,
regardless of the switch state.
To turn off the regulator, the switch is pressed and held.
The controller reads the switch status and pulls ON1
low. The switch is released and ON2 is pulled high.
The MAX849 is an ideal power supply for the power
amplifier (PA) and the radio used in digital cordless
and PCS phones (Figure 10). The PA is directly pow-
ered by the MAX849 for maximum output swing. Post-
linear regulators power the controller and the radio. In
addition, they reduce switching noise and ripple. Table
3 lists the output power available when operating with
one or more NiCd/NiMH cells or one Li-Ion cell.
Figure 10. Typical Phone Application
PA POWER SUPPLY
OUTPUT VOLTAGE:
Power Amplifier (PA) and Radio Supply
(V)
3.3
3.3
5.0
5.0
Using a Momentary On/Off Switch
MAX848
MAX849
C
Applications Information
in a Typical Phone Application
MAX8863/MAX8864 SINGLES
MAX8865/MAX8866 DUALS
RADIO
OUTPUT POWER
(W)
0.9
2.4
2.6
4.3
PA
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