Rainbow Electronics MAX1760 - Manual (Page 8)

MAX1760
0.8A, Low-Noise, 1MHz,
Step-Up DC-DC Converter
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output filter capacitor and the load. As the energy
stored in the inductor is depleted, the current ramps
down and the synchronous rectifier turns off. At light
loads, the device operates at fixed frequency or only as
needed to maintain regulation, depending on the
CLK/SEL setting (Table 2).
Normal Operation
Pulling CLK/SEL low selects the MAX1760’s normal
operating mode. In this mode, the device operates in
PWM when driving medium-to-heavy loads, and auto-
matically switches to PFM if the load requires less
power. PFM operation allows higher efficiency than
PWM under light load conditions.
Forced PWM Operation
When CLK/SEL is high, the MAX1760 operates in a low-
noise PWM-only mode. During forced PWM operation,
the MAX1760 switches at a constant frequency (1MHz),
and modulates the MOSFET switch pulse width to con-
trol the power transferred per cycle to regulate the out-
put voltage. Switching harmonics generated by
fixed-frequency operation are consistent and easily fil-
tered. See the Noise Spectrum plot in the Typical
Operating Characteristics.
Synchronized PWM Operation
The MAX1760 can be synchronized in PWM mode to an
external frequency of 500kHz to 1.2MHz by applying an
external clock signal to CLK/SEL. This allows interfer-
ence to be minimized in wireless applications. The syn-
chronous rectifier is active during synchronized PWM
operation.
Synchronous Rectifier
The MAX1760 features an internal, P-channel synchro-
nous rectifier to enhance efficiency. Synchronous recti-
fication provides 5% improved efficiency over similar
nonsynchronous boost regulators. In PWM mode, the
synchronous rectifier is turned on during the second
half of each switching cycle. In low-power mode, an
internal comparator turns on the synchronous rectifier
when the voltage at LX exceeds the boost regulator
output, and turns it off when the inductor current drops
below 60mA. When setting output voltages greater than
4V, an external 0.5A Schottky diode must be connected
in parallel with the on-chip synchronous rectifier.
Low-Voltage Startup Oscillator
The MAX1760 uses a CMOS, low-voltage startup oscil-
lator for a 1.1V guaranteed minimum startup input volt-
age. At startup, the low-voltage oscillator switches the
N-channel MOSFET until the output voltage reaches
2.15V. Above this level, the normal boost-converter
feedback and control circuitry take over. Once the
device is in regulation, it can operate down to 0.7V
input since internal power for the IC is bootstrapped
from the output through OUT. Do not apply full load
until the output exceeds 2.3V.
Shutdown
The MAX1760 has a shutdown mode that reduces qui-
escent current to 0.1µA. During shutdown (ON = high),
the reference and all feedback and control circuitry are
off. During shutdown, the output voltage is one diode
drop below the input voltage.
Table 1. Typical Available Output Current
5.0 3.6 3 750
3.3 2.4
2
3.3 1.2 1
OUTPUT VOLTAGE
(V)
INPUT VOLTAGE
(V)
NUMBER OF NiCd/NiMH
CELLS
800
350
OUTPUT CURRENT
(mA)
5.0 2.4 500
Table 2. Selecting the Operating Mode
CLK/SEL MODE FEATURES
0 Normal Operation High efficiency at all loads. Fixed frequency at all but light loads.
1 Forced PWM Low noise, fixed frequency at all loads.
External Clock
500kHz to 1.2MHz
Synchronized PWM Low noise, fixed frequency at all loads.