Datasheet

MAX8506/MAX8507/MAX8508
PWM Step-Down DC-DC Converters with 75m
ΩΩ
Bypass FET for WCDMA and cdmaOne Handsets
8 _______________________________________________________________________________________
Synchronous Rectification
An N-channel synchronous rectifier operates during the
second half of each switching cycle (off-time). When
the inductor current falls below the N-channel current-
comparator threshold or when the PWM reaches the
end of the oscillator period, the synchronous rectifier
turns off. This prevents reverse current from the output
to the input in pulse-skipping mode. During PWM oper-
ation, the NEGLIM threshold adjusts to permit reverse
current during light loads. This allows regulation with a
constant switching frequency and eliminates minimum
load requirements for fixed-frequency operation.
Shutdown Mode
Drive SHDN to GND to place the MAX8506/MAX8507/
MAX8508 in shutdown mode. In shutdown, the refer-
ence, control circuitry, internal switching MOSFET, and
synchronous rectifier turn off and the output becomes
high impedance. Input current falls to 0.1µA (typ) dur-
ing shutdown mode. Drive SHDN high to enable the IC.
Current-Sense Comparators
The MAX8506/MAX8507/MAX8508 use several internal
current-sense comparators. In PWM operation, the PWM
comparator terminates the cycle-by-cycle on-time and
provides improved load and line response. A second cur-
rent-sense comparator used across the P-channel switch
controls entry into skip mode. A third current-sense com-
parator monitors current through the internal N-channel
MOSFET to prevent excessive reverse currents 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 P-channel MOSFET detects overcurrent in the
HP mode or at dropout. This protects the system, exter-
nal components, and internal MOSFETs under overload
conditions.
Applications Information
Setting the Output Voltage
Using a DAC (MAX8506/MAX8507)
The MAX8506/MAX8507 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 in many
steps from 3.4V to as low as 0.4V to greatly reduce bat-
tery current (see the Typical Application Circuits). The
use of DC-DC converters such as the MAX8506/
MAX8507 dramatically extends talk time in these appli-
cations.
The MAX8506/MAX8507s’ output voltage is dynamically
adjustable from 0.4V to 3.4V by the use of the REFIN
input. The gain from V
REFIN
to V
OUT
is internally set to
1.76 (MAX8506) or 2.00 (MAX8507). V
OUT
can be adjust-
ed during operation by driving REFIN with an external
DAC. The MAX8506/MAX8507 output responds to full-
scale change in voltage and current in less than 30µs.
Using External Divider (MAX8508)
The MAX8508 is intended for two-step V
CC
control
applications where high efficiency is a priority. Select
an output voltage between 0.75V and 3.4V by connect-
ing FB to a resistive-divider between the output and
GND (see the MAX8508 Typical Application Circuit).
Select feedback resistor R2 in the 5kΩ to 50kΩ range.
R1 is then given by:
where V
FB
= 0.75V.
Input Capacitor Selection
Capacitor ESR is a major contributor to input ripple in
high-frequency DC-DC converters. Ordinary aluminum-
electrolytic capacitors have high ESR and should be
avoided. Low-ESR tantalum or polymer capacitors are
better and provide a compact solution for space-con-
strained surface-mount designs. Ceramic capacitors
have the lowest overall ESR.
The input filter capacitor reduces peak currents and
noise at the input voltage source. Connect a low-ESR
bulk capacitor (2.2µF to 10µF) to the input. Select this
bulk capacitor to meet the input ripple requirements
and voltage rating rather than capacitance value. Use
the following equation to calculate the maximum RMS
input current:
Compensation, Stability, and
Output Capacitor
The MAX8506/MAX8507/MAX8508 are externally com-
pensated by placing a resistor and a capacitor (see the
Typical Application Circuits, R
C
and C
C
) in series from
COMP to GND. An additional capacitor (C
f
) may be
required from COMP to GND if high-ESR output capaci-
tors are used. The C
C
capacitor integrates the current
from the transimpedance amplifier, averaging output
I
I
V
VVV
RMS
OUT
IN
OUT IN OUT
=× ×−()
RR
V
V
OUT
FB
12 1=× −
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