MAX8500 MAXIM [Maxim Integrated Products], MAX8500 Datasheet - Page 10

• Manufacturer Part Number: MAX8500
• Description: PWM Buck Converters with Bypass FET for N-CDMA/W-CDMA Handsets
• Manufacturer: MAXIM [Maxim Integrated Products]

PWM Buck Converters with Bypass FET
for N-CDMA/W-CDMA Handsets
A high-power bypass mode is available on the
MAX8504 for use when a PA transmits at high power.
This mode connects OUT to BATT through the bypass
PFET. Additionally, the PWM buck converter is forced
into 100% duty cycle to further reduce dropout.
Driving SHDN to GND places the MAX8500–MAX8504 in
shutdown mode. In shutdown, the reference, control cir-
cuitry, internal switching MOSFET, and synchronous rec-
tifier turn off and the output becomes high impedance.
Input current falls to 0.1µA (typ) during shutdown mode.
Drive SHDN high for normal operation.
The MAX8500–MAX8504 use several internal current-
sense comparators. In PWM operation, the PWM com-
parator terminates the cycle-by-cycle on-time and
provides improved load and line response. A second
current-sense comparator used across the P-channel
switch controls entry into skip mode. A third current-
sense comparator monitors current through the internal
N-channel MOSFET to prevent excessive reverse cur-
rents and determine when to turn off the synchronous
rectifier. A fourth comparator used at the P-channel
MOSFET detects overcurrent. A fifth comparator used
at the bypass/LDO P-channel MOSFET detects over-
current in the HP mode or at dropout. This protects the
system, external components, and internal MOSFETs
under overload conditions.
The MAX8500–MAX8503 are optimized for highest sys-
tem efficiency when applying power to a linear PA in
CDMA handsets. When transmitting at less than full
power, the supply voltage to the PA is lowered from
V
rent. Figure 3 shows the typical CDMA PA load profile.
The use of DC-to-DC converters such as the MAX8500–
MAX8503 dramatically extends talk time in these appli-
cations.
The MAX8500–MAX8503s’ output voltage is dynamically
adjustable from 0.4V to V
input. The gain from V
1.76X (MAX8500 and MAX8502) or 2X (MAX8501 and
MAX8503). V
10
BATT
______________________________________________________________________________________
to as low as 0.4V to greatly reduce battery cur-
High-Power Bypass Mode (MAX8504)
OUT
Applications Information
can be adjusted during operation by
Using a DAC (MAX8500–MAX8503)
Current-Sense Comparators
Setting the Output Voltage
REFIN
BATT
to V
by the use of the REFIN
OUT
Shutdown Mode
is internally set to
driving REFIN with an external DAC. The MAX8500–
MAX8503 output responds to full-scale change in voltage
and current in <30µs.
The MAX8504 is intended for two-step V
applications where high efficiency is a priority. Select
an output voltage between 1.25V and V
necting FB to a resistive divider between the output
and GND (Figure 4). Select feedback resistor R2 in the
5k to 50k range. R1 is then given by:
where V
Figure 3. Typical W-CDMA Power Amplifier Load Profile
Figure 4. Setting the Adjustable Output Voltage
FB
3.4
3.0
1.0
0.4
= 1.25V.
0
MAX8504
0
30
R
W-CDMA PA SUPPLY CURRENT (mA)
1
Using External Divider (MAX8504)
R
LX
FB
2
V
V
OUT
300
FB
- 1
R1
R2
50k
BATT
600
CC
by con-
control