AN2839 datasheet(29/40 Pages) STMICROELECTRONICS

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AN2839

Application example

Doc ID 15104 Rev 1

29/40

Application example

The first step is to select the type of decay. For the purpose of this example, we will

implement slow decay, which allows a lower power dissipation, lower ripple and avoids

voltages below GND at the output pins during recirculation.

The power dissipation and thermal analysis PractiSPINTM tool allow calculation of the IC

power dissipation and estimation of the junction temperature. The purpose of this section is

to help perform a fast evaluation of the device, package and dissipating copper area

required by the user’s application, and is intended as a good starting point when designing

an application (from the power dissipation and thermal points of view). Software results,

especially thermal results, need to be confirmed on-bench.

The bulk capacitor needs to withstand at least 24 V + 5% + 25%

≅ 32 V. A 50 V capacitor will

be used. Allowing a voltage ripple of 200 mV, the capacitor’s ESR should be lower than 200

mV/0.5 A = 400 m

Ω; the AC current capability should be about 0.5 A.

Providing a reference voltage of 0.5 V, 1-

Ω sensing resistors are needed. In slow decay

mode, the resistors’ power rating is approximately PR ≅ Irms

2 · R

SENSE · D ≅ 0.25 W.

A 1

Ω - 0.25 W - 1% resistor is used. The charge pump uses recommended components

(1N4148 diodes, ceramic capacitors and a 100

Ω resistor to reduce EMI).

R = 18 k

Ω, C = 1.2 nF are connected to the RC pins, obtaining t

OFF ≅ 16 µs. A 5.6 nF

capacitor has been placed on the EN pin, and the pin is driven by the µC through a 100-k

resistor. With these values, if there is a short-circuit between two OUT pins or an OUT pin

and GND, the power DMOS’s turn off after approximately 1 µs, and tDISABLE ≅ 240 µs.

Table 2.

Application data

Value

Rotation speed

300 rpm (fCK = 1 kHz)

Winding peak current

1 A

Maximum ripple

50 mA

Supply voltage

24 V ± 5%

Sequence

Wave mode

Table 3.

Motor data

Value

Winding resistance

6.6

Winding inductance

7.9 mH

Step angle

1.8°/step

Maximum BEMF at 300 rpm

15 V

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