iru3055 International Rectifier Corp., iru3055 Datasheet - Page 8

• Manufacturer Part Number: iru3055
• Description: 5-bit Programmable 3-phase Synchronous Buck Controller Ic -
• Manufacturer: International Rectifier Corp.

IRU3055
APPLICATION INFORMATION
Constant Switching Frequency 3-Phase
Controller
IRU3055 is a 3-phase buck converter controller. For high
current applications, multiple converters are usually con-
nected in parallel to reduce the power capability for each
individual converter as well as alleviate the thermal stress
on each of the power devices. These individual convert-
ers share a common output, but may have different input
sources. Each individual converter operates at the same
switching frequency but at a different phase. As a result,
the effective input current and output current ripple are
much smaller compared with a single-phase converter.
Another benefit will be faster dynamic load responses.
The block diagram of IRU3055 is shown in Figure 2. The
3-phase oscillator provides a constant frequency and the
three PWMs ramp signals with 120 degree phase shift.
The three comparators and three PWM latches will gen-
erate three PWM outputs to the drivers which are built
inside the IC. A typical 3-phase PWM signal is shown in
Figure 4.
Voltage and Current Loop
IRU3055 has three transconductance error amplifiers.
The master Error amplifier is used to regulate the output
voltage. The output voltage can connect directly, or
through a resistor divider, to the Fb pin of the error ampli-
fier. The compensation network at the output of the am-
plifier (Comp Pin) helps to stabilize the voltage loop. The
non-inverting pin of the master amplifier is connected to
the output of the DAC which interfaces with the micro
processor core and determines the desired output volt-
age. Two additional transconductance amplifiers are used
to balance the output inductor current among 3-phases.
8
Figure 4 - The 3-phase PWM signal.
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Output Current Ripple Reduction
One of advantages of the multi-phase converter is that
the output current ripple is significantly reduced. The
current from multiple converters tend to cancel each
other so that the total output current flowing into the
output capacitor is reduced. In this case, the output in-
ductor in each individual buck converter can be selected
smaller to improve the load transient response without
sacrificing the output current ripple. Figure 5 shows a 3-
phase inductor current and current ripple in the capaci-
tor for 12V input 1.5V, 50A, 3-phase buck converter. The
effective output ripple has three times frequency and a
smaller amplitude compared with each individual con-
verter. Figure 6 indicates the total ripple current, as a
function of duty cycle, normalized to the parameter Vo/
(L3Fs) at zero duty cycle.
(Peak to peak current normalized to the Vo/(L3Fs)).
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Figure 6 - Normalized output current across
1
0
Figure 5 - Output inductor currents and
0
Three
phase
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
output capacitor ripple current.
output capacitor.
Single
phase
Two
phase
D
08/13/02
Rev. 1.4
1