Current Model Buck Converter Example LM3495 LM5576 LT3713

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Current Model Buck Converter Example LM3495
LM5576 LT3713
All materials are from National Semiconductor
website available to any readers, Linear
Technology data sheet.
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Current Mode Buck Converter

• LM3075 from National Semiconductor
• The LM3075 is a current mode control, synchronous
  buck controller IC.
• Current mode control assures excellent line and
  load regulation and a wide loop bandwidth for
  fast response to load transients.
• Other features
• cycle by cycle current limit, 1.24V?2 reference
• Application area automotive power supplies and
  distributed power systems

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• Current mode control can be achieved by either
  sensing across the high side NFET or a sense
  resistor.
• The switching frequency can be selected as either
  200kHz or 300kHz from an internal clock.

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Current Mode Problems
Susceptibility to noise on the current signal is
a very common problem, reducing the ability to
process small on-times (large step-down
ratios). As the duty cycle approaches 50
current mode control exhibits sub-harmonic
oscillations. A fixed slope ramp signal (slope
compensation) is generally added to the current
ramp signal. For large step-down applications
with a large Vin and much smaller Vout, this
minimum on-time can present a problem. measuring
current during a very short time.
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Current Mode Problems
Step down switching regulators designed for high
input voltages must control very short minimum
on-times to operate at high frequencies. The
maximum switching frequency and size of the
inductor and output capacitor are function of the
minimum on-time. The on-time of conventional
current mode controllers is limited by current
measurement delays and the leading edge spike on
the current sense signal.
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When the Buck FET turns on and the diode turns
off, a large reverse recovery current flows,
this current can trip the PWM comparator.
Additional filtering and/or leading edge
blanking is necessary to prevent premature
tripping of the PWM. The emulated current
signal is free of noise and turn-on spikes.
Emulated Current Mode
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Conventional Current Mode
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The buck switch current signal we can note that
the signal can be broken down into two parts, a
pedestal and a ramp. When the buck switch
initially turns on the current level jumps to the
same level that was conducting previously in the
diode. By taking a sample-and-hold measurement
of the diode current just before turning on the
buck switch we can establish the pedestal
level. A small current sense resistor is placed
in series with the diode anode to accomplish this
measurement.
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orange color conventional current mode blue
color emulated current mode
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Current Mode Control
From Jian Li Dissertation 2009, Virginia Tech,
currently with Linear Technology, CA
Show a voltage-mode control for comparison
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constant frequency
constant frequency
variable frequency
variable frequency
variable frequency
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variable frequency
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• V2 Control Architecture
• Using the equivalent series resistor (ESR) of the
  output capacitors as the current sensing resistor
• Output voltage is used to generate both the error
  signal and ramp signal
• Load current information, detected on the voltage
  of the ESR, is directly fed back to the PWM
  modulator through inner loop without going
  through any low pass filter or compensation
  network, so that fast transient response can be
  achieved.

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From Saurabh Kasats MS thesis Okalahoma State
University, December 2004