NCP3020A Ver la hoja de datos (PDF) - ON Semiconductor

Número de pieza

componentes Descripción

Fabricante

NCP3020A

Synchronous PWM Controller

ON Semiconductor 

NCP3020A, NCP3020B, NCV3020A, NCV3020B

18

17

16

15

18 V

14

13

12

11

10

9

8

7

6

5

4

3

9V

2

1

0

10%

15%

15 V

12 V

20% 25%

Vout = 3.3 V

30% 35% 40%

VIN, (V)

Figure 32. Ripple Current Ratio vs. Inductance

To keep within the bounds of the parts maximum rating,

calculate the RMS current and peak current.

Ǹ IRMS + IOUT @

1 ) ra2 ³ 10.02 A

12

Ǹ + 10 A @ 1 ) (0.24)2

12

(eq. 8)

ǒ Ǔ ǒ Ǔ IPK + IOUT @

1 ) ra

2

³ 11.2 A + 10 A @

(0.24)

1)

2

(eq. 9)

An inductor for this example would be around 3.3 mH and

should support an rms current of 10.02 A and a peak current

of 11.2 A.

The final selection of an output inductor has both

mechanical and electrical considerations. From a

mechanical perspective, smaller inductor values generally

correspond to smaller physical size. Since the inductor is

often one of the largest components in the regulation system,

a minimum inductor value is particularly important in

space−constrained applications. From an electrical

perspective, the maximum current slew rate through the

output inductor for a buck regulator is given by Equation 10.

SlewRateLOUT

+

VIN * VOUT

LOUT

³

2.6

A

ms

12 V * 3.3 V

+

3.3 mH

(eq. 10)

This equation implies that larger inductor values limit the

regulator’s ability to slew current through the output

inductor in response to output load transients. Consequently,

output capacitors must supply the load current until the

inductor current reaches the output load current level. This

results in larger values of output capacitance to maintain

tight output voltage regulation. In contrast, smaller values of

inductance increase the regulator’s maximum achievable

slew rate and decrease the necessary capacitance, at the

expense of higher ripple current. The peak−to−peak ripple

current for the NCP3020A is given by the following

equation:

IPP

+

VOUT(1

LOUT @

* D)

FSW

(eq. 11)

Ipp is the peak to peak current of the inductor. From this

equation it is clear that the ripple current increases as LOUT

decreases, emphasizing the trade−off between dynamic

response and ripple current.

The power dissipation of an inductor consists of both

copper and core losses. The copper losses can be further

categorized into dc losses and ac losses. A good first order

approximation of the inductor losses can be made using the

DC resistance as they usually contribute to 90% of the losses

of the inductor shown below:

LPCU + IRMS 2 @ DCR

(eq. 12)

The core losses and ac copper losses will depend on the

geometry of the selected core, core material, and wire used.

Most vendors will provide the appropriate information to

make accurate calculations of the power dissipation then the

total inductor losses can be capture buy the equation below:

LPtot + LPCU_DC ) LPCU_AC ) LPCore (eq. 13)

Input Capacitor Selection

The input capacitor has to sustain the ripple current

produced during the on time of the upper MOSFET, so it

must have a low ESR to minimize the losses. The RMS value

of this ripple is:

IinRMS + IOUT @ ǸD @ (1 * D)

(eq. 14)

D is the duty cycle, IinRMS is the input RMS current, and

IOUT is the load current.

The equation reaches its maximum value with D = 0.5.

Loss in the input capacitors can be calculated with the

following equation:

PCIN + ESRCIN @ ǒIIN*RMSǓ2

(eq. 15)

PCIN is the power loss in the input capacitors and ESRCIN

is the effective series resistance of the input capacitance.

Due to large dI/dt through the input capacitors, electrolytic

or ceramics should be used. If a tantalum must be used, it

must by surge protected. Otherwise, capacitor failure could

occur.

Input Start−up Current

To calculate the input startup current, the following

equation can be used.

IINRUSH

+

COUT @ VOUT

tSS

(eq. 16)

Iinrush is the input current during startup, COUT is the total

output capacitance, VOUT is the desired output voltage, and

tSS is the soft start interval. If the inrush current is higher than

the steady state input current during max load, then the input

fuse should be rated accordingly, if one is used.

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