AN723 データシートの表示（PDF） - Vishay Semiconductors

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AN723

Design A High Performance Buck or Boost Converter With Si9165

Vishay Semiconductors 

AN723

Vishay Siliconix

FIGURE 6. Buck Converter Loop Gain with Type I

Compensation

FIGURE 7. Buck Converter Loop Gain with Type III

Compensation

FIGURE 9. Buck Load Transient w/Type III Compensation

VIN = 3.6 V, VOUT = 2.64 V, 300 mA Load Transient

For voltage mode control, a simple Type I or II compensation

network can easily stabilize the loop but at a cost of lower BW,

which has to be at least one decade below the L-C corner

frequency to preserve a good stability margin. However, Type

III compensation, a more complicated design, enables higher

BW even above the L-C double-pole. A buck converter is used

as an example here to illustrate the difference between Type I

(Figure 6) and Type III (Figure 7) compensation. With the

switching frequency set to 2 MHz, a 1.5-µH to 10-µF L-C pair

is used for the power stage, producing a double pole at

40 kHz. The loop gain Bode plots are measured for both types

under the same conditions: VIN = 3.6 V, VOUT = 2.7 V, load =

300 mA. (See Figures 8 and 9). The BW is considerably

higher with Type III compensation. The resulting transient

waveforms for the two loops (Figures 10 and 11) show a

notable improvement in both over/undershoot magnitude and

recovery time with Type III compensation.

The values shown in Figure 7 work well for the Si9165 buck

converter with a 3.6-V input, so long as the switching

frequency is above 500 kHz, which is the range Si9165 is

optimized for. Since the converter power stage gain varies

with input voltage, the compensation circuit needs to be

adjusted accordingly to maintain a stability margin. The

circuitry in Figure 7 offers fast response with a sufficient

stability margin for input voltages below 3.6 V. If the input

voltage is above 3.6 V, the power stage gain, also part of the

loop gain, elevates to a level that will endanger both the phase

margin and gain margin of the control loop. Fortunately for

buck converters, a simple change on the input lead capacitor

C3 can help compensate for this. The value can be adjusted

by the simple equation shown below:

C3

=

3----.-6----V------⋅---2---7---0----p----F--

VIN

(5)

FIGURE 8. Buck Load Transient w/Type I Compensation

VIN = 3.61 V, VOUT = 2.676 V, 300 mA Load Transient

FaxBack 408-970-5600, request 70823

www.siliconix.com

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