Rainbow Electronics MAX1706 - Manual (Page 11)

MAX1705/MAX1706
1- to 3-Cell, High-Current, Low-Noise,
Step-Up DC-DC Converters with Linear Regulator
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Step-Up Converter
The step-up switching DC-DC converter generates an
adjustable output to supply both power circuitry (such
as RF power amplifiers) and the internal low-dropout
linear regulator. During the first part of each cycle, the
internal N-channel MOSFET switch is turned on. This
allows current to ramp up in the inductor and store
energy in a magnetic field. During the second part of
each cycle, when the MOSFET is turned off, the voltage
across the inductor reverses and forces current
through the diode and synchronous rectifier to the out-
put filter capacitor and load. As the energy stored in
the inductor is depleted, the current ramps down, and
the output diode and synchronous rectifier turn off.
Voltage across the load is regulated using either PWM
or PFM operation, depending on the CLK/SEL pin set-
ting (Table 1).
Low-Noise, High-Power PWM Operation
When CLK/SEL is pulled high, the MAX1705/MAX1706
operate in a high-power, low-noise PWM mode. During
PWM operation, they switch at a constant frequency
(300kHz), and modulate the MOSFET switch pulse
width to control the power transferred per cycle and
regulate the voltage across the load. In PWM mode, the
devices can output up to 850mA. Switching harmonics
generated by fixed-frequency operation are consistent
and easily filtered.
During PWM operation, each of the internal clock’s ris-
ing edges sets a flip-flop, which turns on the N-channel
MOSFET switch (Figure 3). The switch is turned off
when the sum of the voltage-error and current-
feedback signals trips a multi-input comparator and
resets the flip-flop; the switch remains off for the rest of
the cycle. When a change occurs in the output voltage
error signal into the comparator, it shifts the level that
the inductor current is allowed to ramp to during each
cycle and modulates the MOSFET switch pulse width.
A second comparator enforces a 1.55A (max) inductor-
LX
POUT
OUT
BOOST OUTPUT 3.6V
FB
LDO OUTPUT 3.3V
INPUT 0.9V TO 3.6V
(TO PGND)
(TO PGND)
PGND
GND
LDO
FBLDO
LBO
LBN
REF
MAX1705
MAX1706
LBP
CLK/SEL
ONA
ONB
TRACK
R3
165k
R4
100k
R5
R6
R7
100k
C2*
D1
C7
22
µ F
C8
0.33 µ F
L1 10 µ H (22 µ H)
*OPTIONAL.
( ) ARE FOR MAX1706.
C4
220
µ F
(100 µ F)
C5*
0.33
µ F
C6
22
µ F
R1
191k
C3
0.1
µ F
C9
0.33
µ F
C1*
R2
100k
NOTE: HEAVY LINES INDICATE HIGH-CURRENT PATH.
Figure 2. Typical Operating Circuit (PWM Mode)
CLK/SEL MODE FEATURES
0 PFM Low supply current
1 PWM
Low noise,
high output current
External Clock
(200kHz to 400kHz)
Synchronized
PWM
Low noise,
high output current
Table 1. Selecting the Operating Mode