Download Current Model Buck Converter Example LM3495 LM5576

Current Model Buck Converter
Example LM3495 LM5576 LT3713
All materials are from National Semiconductor website
available to any readers, Linear Technology data sheet.
1
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
2
• 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.
3
Current Mode Problems
Susceptibility to noise on the current signal is a very
common problem, reducing the ability to process small ontimes (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.
4
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.
5
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 turnon spikes.
Emulated Current Mode
6
Conventional Current Mode
7
8
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.
9
10
orange color: conventional current mode
blue color: emulated current mode
11
Current Mode Control
From Jian Li Dissertation 2009, Virginia
Tech, currently with Linear Technology, CA
Show a voltage-mode control for comparison
12
13
constant frequency
constant frequency
variable frequency
variable frequency
14
variable frequency
15
16
17
18
V2 Control Architecture
1. Using the equivalent series resistor (ESR) of the output capacitors
as the current sensing resistor
2. Output voltage is used to generate both the error signal and ramp
signal
3. 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.
19
From Saurabh Kasat’s MS thesis
Okalahoma State University,
December 2004
20