Block Diagram
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General Description
The LMR64010 switching regulators is a current-mode boost
converter operating at a fixed frequency of 1.6 MHz.
The use of SOT-23 package, made possible by the minimal
power loss of the internal 1A switch, and use of small induc-
tors and capacitors result in the industry's highest power
density. The 40V internal switch makes these solutions per-
fect for boosting to voltages of 16V or greater.
These parts have a logic-level shutdown pin that can be used
to reduce quiescent current and extend battery life.
Protection is provided through cycle-by-cycle current limiting
and thermal shutdown. Internal compensation simplifies de-
sign and reduces component count.
Theory of Operation
The LMR64010 is a switching converter IC that operates at a
fixed frequency (1.6 MHz) using current-mode control for fast
transient response over a wide input voltage range and in-
corporates pulse-by-pulse current limiting protection. Be-
cause 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 current
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.
Application Hints
SELECTING THE EXTERNAL CAPACITORS
The best capacitors for use with the LMR64010 are multi-lay-
er 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.
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
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LMR64010