LM5000 datasheet(12/23 Pages) TI1

12 / 23 page

fPC2 =

2SCC2(RC //RO)

(in Hz)

fZC =

2SCCRC

(in Hz)

fPC =

2S(RC + RO)CC

(in Hz)

RHPzero =

VOUT(D')

2S,LOADL

LM5000

SNVS176D – MAY 2004 – REVISED MARCH 2007

www.ti.com

RIGHT HALF PLANE ZERO

A current mode control boost regulator has an inherent right half plane zero (RHP zero). This zero has the effect

of a zero in the gain plot, causing an imposed +20dB/decade on the rolloff, but has the effect of a pole in the

phase, subtracting another 90° in the phase plot. This can cause undesirable effects if the control loop is

influenced by this zero. To ensure the RHP zero does not cause instability issues, the control loop should be

designed to have a bandwidth of ½ the frequency of the RHP zero or less. This zero occurs at a frequency of:

(10)

where ILOAD is the maximum load current.

SELECTING THE COMPENSATION COMPONENTS

The first step in selecting the compensation components RC and CC is to set a dominant low frequency pole in

the control loop. Simply choose values for RC and CC within the ranges given in the Introduction to

Compensation section to set this pole in the area of 10Hz to 100Hz. The frequency of the pole created is

determined by the equation:

(11)

where RO is the output impedance of the error amplifier, 850kΩ. Since RC is generally much less than RO, it does

not have much effect on the above equation and can be neglected until a value is chosen to set the zero fZC. fZC

is created to cancel out the pole created by the output capacitor, fP1. The output capacitor pole will shift with

different load currents as shown by the equation, so setting the zero is not exact. Determine the range of fP1 over

the expected loads and then set the zero fZC to a point approximately in the middle. The frequency of this zero is

determined by:

(12)

Now RC can be chosen with the selected value for CC. Check to make sure that the pole fPC is still in the 10Hz to

100Hz range, change each value slightly if needed to ensure both component values are in the recommended

range. After checking the design at the end of this section, these values can be changed a little more to optimize

performance if desired. This is best done in the lab on a bench, checking the load step response with different

values until the ringing and overshoot on the output voltage at the edge of the load steps is minimal. This should

produce a stable, high performance circuit. For improved transient response, higher values of RC (within the

range of values) should be chosen. This will improve the overall bandwidth which makes the regulator respond

more quickly to transients. If more detail is required, or the most optimal performance is desired, refer to a more

in depth discussion of compensating current mode DC/DC switching regulators.

HIGH OUTPUT CAPACITOR ESR COMPENSATION

When using an output capacitor with a high ESR value, or just to improve the overall phase margin of the control

loop, another pole may be introduced to cancel the zero created by the ESR. This is accomplished by adding

another capacitor, CC2, directly from the compensation pin VC to ground, in parallel with the series combination of

RC and CC. The pole should be placed at the same frequency as fZ1, the ESR zero. The equation for this pole

follows:

(13)

To ensure this equation is valid, and that CC2 can be used without negatively impacting the effects of RC and CC,

fPC2 must be greater than 10fPC.

CHECKING THE DESIGN

The final step is to check the design. This is to ensure a bandwidth of ½ or less of the frequency of the RHP

zero. This is done by calculating the open-loop DC gain, ADC. After this value is known, you can calculate the

crossover visually by placing a

−20dB/decade slope at each pole, and a +20dB/decade slope for each zero. The

point at which the gain plot crosses unity gain, or 0dB, is the crossover frequency. If the crossover frequency is

at less than ½ the RHP zero, the phase margin should be high enough for stability. The phase margin can also

be improved some by adding CC2 as discussed earlier in the section. The equation for ADC is given below with

additional equations required for the calculation:

Submit Documentation Feedback

Product Folder Links: LM5000

LM5000 Datasheet(PDF) 12 Page - Texas Instruments

LM5000 Datasheet(HTML) 12 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL