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LM20136 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 11 Page - Texas Instruments

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๋ถ€ํ’ˆ๋ช… LM20136
์ƒ์„ธ๋‚ด์šฉ  LM20136 6A, Synchronous Buck Regulator with Input Synchronization
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LM20136
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SNVS564B โ€“ JANUARY 2009 โ€“ REVISED APRIL 2013
OPERATION DESCRIPTION
General
The LM20136 switching regulator features all of the functions necessary to implement an efficient low voltage
buck regulator using a minimum number of external components. This easy to use regulator features two
integrated switches and is capable of supplying up to 6A of continuous output current. The regulator utilizes peak
current mode control with nonlinear slope compensation to optimize stability and transient response over the
entire output voltage range. Peak current mode control also provides inherent line feed-forward, cycle-by-cycle
current limiting and easy loop compensation. The internal oscillator can synchronize up to 1.5 MHz minimizing
the inductor size while still achieving efficiencies up to 96%. The precision internal voltage reference allows the
output to be set as low as 0.8V. Fault protection features include: current limiting, thermal shutdown, over voltage
protection, and shutdown capability. The device is available in the HTSSOP package featuring an exposed pad
to aid thermal dissipation. The LM20136 can be used in numerous applications to efficiently step-down from a 5V
or 3.3V bus. The typical application circuit for the LM20136 is shown in Figure 30 in the design guide.
Precision Enable
The enable (EN) pin allows the output of the device to be enabled or disabled with an external control signal.
This pin is a precision analog input that enables the device when the voltage exceeds 1.18V (typical). The EN pin
has 66 mV of hysteresis and will disable the output when the enable voltage falls below 1.11V (typical). If the EN
pin is not used, it should be connected to VIN. Since the enable pin has a precise turn on threshold it can be
used along with an external resistor divider network from VIN to configure the device to turn on at a precise input
voltage. The precision enable circuitry will remain active even when the device is disabled.
Frequency Synchronization
The frequency synchronization pin (SYNC) allows the switching frequency of the device to be controlled with an
external clock signal. This feature allows the user to synchronize multiple converters, avoiding undesirable
frequency bands of operation. Multiple devices can be configured to switch out of phase reducing input capacitor
requirements and EMI of the power supply system.
The turn on of the high-side switch will lock-on to the rising edge of the SYNC pin input. The logic low level for
the input clock must be below 0.8V and the logic high level must exceed 2.0V to ensure proper operation. The
device will synchronize to frequencies from 500 kHz to 1.5 MHz. If the synchronization clock is removed or not
present during startup, the oscillator of the device will run at approximately 410 kHz. If the SYNC pin is not used
it should be connected to ground.
Peak Current Mode Control
In most cases, the peak current mode control architecture used in the LM20136 only requires two external
components to achieve a stable design. The compensation can be selected to accommodate any capacitor type
or value. The external compensation also allows the user to set the crossover frequency and optimize the
transient performance of the device.
For duty cycles above 50% all current mode control buck converters require the addition of an artificial ramp to
avoid sub-harmonic oscillation. This artificial linear ramp is commonly referred to as slope compensation. What
makes the LM20136 unique is the amount of slope compensation will change depending on the output voltage.
When operating at high output voltages the device will have more slope compensation than when operating at
lower output voltages. This is accomplished in the LM20136 by using a non-linear parabolic ramp for the slope
compensation. The parabolic slope compensation of the LM20136 is much better than the traditional linear slope
compensation because it optimizes the stability of the device over the entire output voltage range.
Current Limit
The precise current limit of the LM20136 is set at the factory to be within 10% over the entire operating
temperature range. This enables the device to operate with smaller inductors that have lower saturation currents.
When the peak inductor current reaches the current limit threshold, an over current event is triggered and the
internal high-side FET turns off and the low-side FET turns on allowing the inductor current to ramp down until
the next switching cycle. For each sequential over-current event, the reference voltage is decremented and PWM
pulses are skipped resulting in a current limit that does not aggressively fold back for brief over-current events,
while at the same time providing frequency and voltage foldback protection during hard short circuit conditions.
Copyright ยฉ 2009โ€“2013, Texas Instruments Incorporated
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