MP2235 datasheet(14/21 Pages) MPS

14 / 21 page

MP2235

– SYNCHRONOUS STEP-DOWN CONVERTER

crossover frequency below one-tenth of the

switching frequency.

To optimize the compensation components, the

following procedure can be used.

1. Choose high-side feedback resistor R1 and

calculate the value of low-side resistor R2

according to desired output voltage. Suggest

choosing R1 around 40kΩ for >1.2V output

condition.

2. Choose the T-Type resistor RT to set the

desired crossover frequency. Determine the RT

value by the following equation:

OUT





   



RZ is the internal compensation resistor, which

equals to 300kΩ. f

is the desired crossover

frequency which is typically one tenth of the

switching frequency. Ri is the current sense

resistance, 0.22Ω.

3. Choose feed forward capacitor CF to achieve

sufficient phase margin especially for large

output inductor condition. In theory there is no

need to add type III zero for peak current mode

control, but in real circuit there are some parasitic

capacitors or filters internal which induces poles

into the control loop. Fortunately, those poles are

locating at high frequency range which won’t

affect the step 2 bandwidth calculation while it

affects the phase margin. For applications with

typical inductor values (<4.7µH), setting the

compensation zero, fZ3 (formed by R1 and CF)

around 1.5 times of crossover frequency fC. Then

the CF value can be calculated by following

equation:







If electrolytic capacitor is used or the output

capacitor has large ESR, the feed forward

capacitor CF is not needed any more since there

is already one ESR zero in the loop.

If large output inductor is used, like 22µH, the

phase margin will decrease a lot due to the half

switching

frequency

pole

moves

towards

crossover

frequency.

is condition, it’s

suggested increasing feed forward capacitor

value of CF to get enough phase margins while

it’s better to keep the feed forward zero

frequency

higher

than

half

crossover

frequency.

Figure 7: T-Type Network

Table 1 lists the recommended resistors and

compensation

values

for

common

output

voltages (refer to Figure 7).

Table 1: Resistor Selection for Common Output

Voltages

VOUT

(V)

R1 (kΩ)

R2 (kΩ) Rt (kΩ) Cf(pF) L(μH)

20.5

84.5

1.2

30.1

61.9

1.8

40.2

32.4

2.2

2.5

40.2

19.1

6.8

2.2

3.3

40.2

5.6

3.3

40.2

7.68

3.3

For more accurate control loop design, visit MPS

website and run the online bode plot simulation

by DC/DC designer.

Selecting the Inductor

Use a 1µH-to-22µH inductor with a DC current

rating of at least 25% percent higher than the

maximum load current for most applications. For

highest efficiency, use an inductor with a DC

resistance

less than 15mΩ. For most designs,

the inductance value can be derived from the

following equation.

OUT

OSC

)





  

Where ΔI

L is the inductor ripple current.

MP2235 Datasheet(PDF) 14 Page - Monolithic Power Systems

MP2235 Datasheet(HTML) 14 Page - Monolithic Power Systems

Html Pages

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

Datasheet Download

Link URL