MAX1765EEE-T Maxim Integrated Products, MAX1765EEE-T Datasheet - Page 12

• Manufacturer Part Number: MAX1765EEE-T
• Description: DC/DC Switching Converters 800mA Step-Up w/500mA Linear Reg
• Manufacturer: Maxim Integrated Products

The MAX1765 transitions to fixed-frequency PWM oper-
ation under medium and heavy loads. The N-channel
FET is engaged when V
ramp up the current in the inductor until one of the fol-
lowing conditions occurs: the system needs are met,
the next falling edge of the internal oscillator is
achieved, or the maximum inductor current (ISET) is
reached. The N-channel is turned off, activating the P-
channel synchronous rectifier that remains on until the
inductor current gets to the P-channel turn-off current
level, or V
clock edge. The 1MHz fixed-frequency operation pro-
duces an easily filtered fixed-noise spectrum.
When CLK/SEL is high, the MAX1765 operates in a low-
noise PWM-only mode. The N-channel FET is turned on
when V
tor current until one of the following conditions occurs:
the system needs are met, the next falling edge of the
internal oscillator is achieved, or the ISET is reached.
The N-channel is then turned off, activating the P-chan-
nel synchronous rectifier that remains on until the next
rising edge of the oscillator, where the N-channel is
again turned on under most conditions. The P-channel
800mA, Low-Noise, Step-Up DC-DC Converter
with 500mA Linear Regulator
Figure 2. Typical Application Circuit
12
LOGIC INPUTS
______________________________________________________________________________________
V
0.7V to 3.2V
IN
0.22µF
FB
< V
FB
REF
< V
ILIM
REF
ONA
ONB
TRACK
ONL
GND
REF
and is kept on to ramp up the induc-
MAX1765
CLK/SEL
33µF
PWM Operation in Normal Mode
and there is a rising oscillator
3.3µH
FB
Forced PWM Operation
PGND
POUT
OUTL
OUT
FBL
INL
LX
FB
< V
REF
0.68µF
and is kept on to
4.7Ω
165k
100k
V
4.7µF
OUTL
2.85V
100µF
V
=
OUT
3.3V
zero detect circuitry is deactivated in forced PWM
mode. This means an N- or P-channel FET is on all the
time for most load conditions.
At light loads, the P-channel will remain on so the
device can pass current back to the input from the out-
put. The P-channel will only pass current for two cycles
before it is disabled. Then, the device remains inactive
until V
During forced PWM operation, the MAX1765 switches
at a constant frequency (1MHz) and modulates the
MOSFET switch pulse width to control the power trans-
ferred per cycle in order to regulate the output voltage
for most output currents. Switching harmonics generat-
ed by fixed-frequency operation are consistent and
easily filtered. (See the Boost Followed by LDO Output
Noise Spectrum plot in the Typical Operating
Characteristics.)
The MAX1765 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 inter-
ference to be minimized in wireless applications. The
synchronous rectifier is active during synchronized
PWM operation.
The MAX1765 features an internal 250mΩ, P-channel
synchronous rectifier to enhance efficiency. Synchronous
rectification provides a 5% efficiency improvement over
similar nonsynchronous boost regulators. In PWM mode,
the synchronous rectifier is turned on during the second
portion of each switching cycle. At light loads (in normal
mode), an internal comparator turns on the synchronous
rectifier when the voltage at LX exceeds the boost regu-
lator output, and turns it off when the inductor current
drops below 50mA.
The MAX1765 uses a low-voltage startup oscillator for a
1.1V guaranteed minimum input startup input voltage.
A Schottky diode placed across LX and POUT reduces
the startup voltage to 0.9V. At startup, the low-voltage
oscillator switches the N-channel MOSFET until the out-
put voltage reaches 2.15V. Above this level, the normal
boost-converter feedback and control circuitry takes
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 OUT pin. Do not apply full load
until the output exceeds 2.3V.
The MAX1765 contains an LDO with a fixed 2.85V (or
adjustable) output. The MAX1765 linear regulator fea-
FB
< V
REF
Low-Voltage Startup Oscillator
.
Synchronized PWM Operation
Synchronous Rectifier
Linear Regulator