1- to 3-Cell, High-Current, Low-Noise,
Step-Up DC-DC Converters with Linear Regulator
MAX1705/MAX1706
POUT
Q
IFB*
IREF*
R
Q
N
LX
IFB*
ICS
IREF*
CURRENT
LIMIT LEVEL
OSC
PGND
CURRENT
LIMIT LEVEL
PGND
*SEE FIGURE 1
R
S
Q
N
LX
D
LOGIC HIGH
POUT
R
P
Q
P
S
Figure 3. Simplified PWM Controller Block Diagram
*SEE FIGURE 1
current limit for the MAX1705, and 950mA (max) for the
MAX1706. During PWM operation, the circuit operates
with a continuous inductor current.
Figure 4. Controller Block Diagram in PFM Mode
Synchronized PWM Operation
The MAX1705/MAX1706 can also be synchronized to a
200kHz to 400kHz frequency by applying an external
clock to CLK/SEL. This allows the user to set the har-
monics, to avoid IF bands in wireless applications. The
synchronous rectifier is also active during synchronized
PWM operation.
Low-Power PFM Operation
Pulling CLK/SEL low places the MAX1705/MAX1706 in
low-power standby mode. During standby mode, PFM
operation regulates the output voltage by transferring a
fixed amount of energy during each cycle, and then
modulating the switching frequency to control the
power delivered to the output. The devices switch only
as needed to service the load, resulting in the highest
possible efficiency at light loads. Output current capa-
bility in PFM mode is 140mA (from 2.4V input to 3.3V
output). The output is regulated at 1.3% above the
PWM threshold.
During PFM operation, the error comparator detects
output voltage falling out of regulation and sets a
flip-flop, turning on the N-channel MOSFET switch
(Figure 4). When the inductor current ramps to the PFM
mode current limit (435mA) and stores a fixed amount
of energy, the current-sense comparator resets a flip-
flop. The flip-flop turns off the N-channel switch and
turns on the P-channel synchronous rectifier. A second
flip-flop, previously reset by the switch’s “on” signal,
inhibits the error comparator from initiating another
12
cycle until the energy stored in the inductor is dumped
into the output filter capacitor and the synchronous rec-
tifier current ramps down to 70mA. This forces opera-
tion with a discontinuous inductor current.
Synchronous Rectifier
The MAX1705/MAX1706 feature an internal 270mΩ,
P-channel synchronous rectifier to enhance efficiency.
Synchronous rectification provides a 5% efficiency
improvement over similar nonsynchronous step-up
regulators. In PWM mode, the synchronous rectifier is
turned on during the second half of each cycle. In PFM
mode, an internal comparator turns on the synchronous
rectifier when the voltage at LX exceeds the step-up
converter output, and then turns it off when the inductor
current drops below 70mA.
Linear Regulator
The internal low-dropout linear regulator steps down the
output from the step-up converter and reduces switching
ripple. It is intended to power noise-sensitive analog cir-
cuitry, such as low-noise amplifiers and IF stages in cel-
lular phones and other instruments, and can deliver up to
200mA. However, in practice, the maximum output cur-
rent is further limited by the current available from the
boost converter and by the voltage differential between
OUT and LDO. Use a 22µF capacitor with a 1Ω or less
equivalent series resistance (ESR) at the output for sta-
bility (see the Linear Regulator Region of Stable C6 ESR
vs. Load Current graph in the
Typical Operating
Characteristics).
During power-up, the linear regulator
remains off until the step-up converter goes into regula-
tion for the first time.
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