LM2733YEVAL National Semiconductor, LM2733YEVAL Datasheet - Page 8

• Manufacturer Part Number: LM2733YEVAL
• Description: BOARD EVALUATION LM2733Y
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2733YEVAL

• Main Purpose: DC/DC, Step Up
• Outputs And Type: 1, Non-Isolated
• Current - Output: 1A
• Voltage - Input: 2.7 ~ 14V
• Regulator Topology: Boost
• Frequency - Switching: 600kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2733
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Voltage - Output: -
• Power - Output: -

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Block Diagram
Theory of Operation
The LM2733 is a switching converter IC that operates at a
fixed frequency (0.6 or 1.6 MHz) using current-mode control
for fast transient response over a wide input voltage range
and incorporate pulse-by-pulse current limiting protection.
Because this is current mode control, a 50 mΩ sense resistor
in series with the switch FET is used to provide a voltage
(which is proportional to the FET current) to both the input of
the pulse width modulation (PWM) comparator and the cur-
rent limit amplifier.
At the beginning of each cycle, the S-R latch turns on the FET.
As the current through the FET increases, a voltage (propor-
tional to this current) is summed with the ramp coming from
the ramp generator and then fed into the input of the PWM
comparator. When this voltage exceeds the voltage on the
other input (coming from the Gm amplifier), the latch resets
and turns the FET off. Since the signal coming from the Gm
amplifier is derived from the feedback (which samples the
voltage at the output), the action of the PWM comparator
constantly sets the correct peak current through the FET to
keep the output volatge in regulation.
Q1 and Q2 along with R3 - R6 form a bandgap voltage refer-
ence used by the IC to hold the output in regulation. The
currents flowing through Q1 and Q2 will be equal, and the
feedback loop will adjust the regulated output to maintain this.
Because of this, the regulated output is always maintained at
a voltage level equal to the voltage at the FB node "multiplied
up" by the ratio of the output resistive divider.
The current limit comparator feeds directly into the flip-flop,
that drives the switch FET. If the FET current reaches the limit
threshold, the FET is turned off and the cycle terminated until
the next clock pulse. The current limit input terminates the
pulse regardless of the status of the output of the PWM com-
parator.
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Application Hints
SELECTING THE EXTERNAL CAPACITORS
The best capacitors for use with the LM2733 are multi-layer
ceramic capacitors. They have the lowest ESR (equivalent
series resistance) and highest resonance frequency which
makes them optimum for use with high frequency switching
converters.
When selecting a ceramic capacitor, only X5R and X7R di-
electric types should be used. Other types such as Z5U and
Y5F have such severe loss of capacitance due to effects of
temperature variation and applied voltage, they may provide
as little as 20% of rated capacitance in many typical applica-
tions. Always consult capacitor manufacturer’s data curves
before selecting a capacitor. High-quality ceramic capacitors
can be obtained from Taiyo-Yuden, AVX, and Murata.
SELECTING THE OUTPUT CAPACITOR
A single ceramic capacitor of value 4.7 µF to 10 µF will provide
sufficient output capacitance for most applications. For output
voltages below 10V, a 10 µF capacitance is required. If larger
amounts of capacitance are desired for improved line support
and transient response, tantalum capacitors can be used in
parallel with the ceramics. Aluminum electrolytics with ultra
low ESR such as Sanyo Oscon can be used, but are usually
prohibitively expensive. Typical AI electrolytic capacitors are
not suitable for switching frequencies above 500 kHz due to
significant ringing and temperature rise due to self-heating
from ripple current. An output capacitor with excessive ESR
can also reduce phase margin and cause instability.
SELECTING THE INPUT CAPACITOR
An input capacitor is required to serve as an energy reservoir
for the current which must flow into the coil each time the
switch turns ON. This capacitor must have extremely low
ESR, so ceramic is the best choice. We recommend a nomi-
nal value of 2.2 µF, but larger values can be used. Since this
capacitor reduces the amount of voltage ripple seen at the
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