MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER

MP1586 Rev. 0.91

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13

1/8/2010

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the crossover frequency to approximately one-

tenth of the switching frequency.

The Table 3 lists the typical values of

compensation components for some standard

output voltages with various output capacitors

and inductors. The values of the compensation

components have been optimized for fast

transient responses and good stability at given

conditions.

Table 3—Compensation Values for Typical

Output Voltage/Capacitor Combinations

(V)

L

(µH)

C2

(µF)

R3

(kΩ)

C3

(pF)

C6

1.8

10

2×22

24

820

None

2.5

15

2×22

36

680

None

3.3

15

2×22

47

680

None

5

22

2×22

47

820

None

12

33

2×22

100

820

None

To optimize the compensation components, the

following procedure can be used.

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

2. Choose the compensation capacitor (C3) to

achieve

the

desired phase

margin.

For

applications with typical inductor values, setting

the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C

f

3

R

2

4

3

C

×

×

π

>

3. Determine if the second compensation

capacitor (C6) is required. It is required if the

ESR zero of the output capacitor is located at

less than half of the switching frequency, or the

following relationship is valid:

2

f

R

2

C

2

1

S

ESR

<

×

×

π

If this is the case, then add the second

compensation capacitor (C6) to set the pole fP3

at the location of the ESR zero. Determine the

C6 value by the equation:

3

R

R

2

C

6

C

ESR

×

=

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when the input voltage is no

greater than 5V or the 5V rail is available in the

system. This helps improve the efficiency of the

regulator. The bootstrap diode can be a low

cost one such as IN4148 or BAT54.

MP1586

SW

BS

5V

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty

At no load or light load, the converter may

operate in pulse skipping mode in order to

maintain the output voltage in regulation. Thus

there is less time to refresh the BS voltage. In

order to have enough gate voltage under such

should be greater than 3V. For example, if the

than 3.3V+3V=6.3V to maintain enough BS

voltage at no load or light load. To meet this

requirement, EN pin can be used to program

the input UVLO voltage to Vout+3V.