12. External Resistors

• R4 and R5

The resistors R4 and R5 exist to apply a high-level input to the F/R pin. Since the F/R input has a pull-down

resistor of about 50 k

Ω, it is at the low level when open. Apply a voltage of over 4.0 V and under 6.3 V to input a

high level.

• R15

The resistor R15 exists to apply a high-level input to the S/S pin. Since the S/S input has a pull-down resistor of

about 50 k

Ω, it is at the low level when open. Apply a voltage of over 4.0 V and under 6.3 V for the start state high

level input. Although dividing the voltage with two resistors, as is done with the F/R input, would improve the

resistance to noise since a lower input impedance can be set up, when noise is not a problem the high level can be

set by connecting a single resistor such as R15. A value of 180 k

Ω is recommended.

mode). This is because the S/S pin input voltage is resistor divided and the input voltage will be under 2.6 V (the

13. Through Currents due to the Direct PWM Scheme

In the direct PWM scheme, through currents flow in the outputs due to transistor switching in applications

implemented with either discrete components or the LB1822. This is due to output transistor delays and parasitic

capacitances. Previously, when this was a problem, additional capacitors were used to resolve the problem. However,

since the LB1921 resolves this problem at the circuit level, no additional external components are required. During

switching, whiskers of less than about 10 ns may be observed on the RF voltage waveform, but these are not a

problem.

14. Oscillator Element

Normally, a crystal oscillator is used with this IC. If the speed control characteristic requirements are not stringent, a

ceramic oscillator could be used. To avoid problems, consult the manufacturer of the oscillator element when

selecting the oscillator element and determining the values of the external resistors and capacitors.

Start mode:

Stop mode:

• Power dissipation when –10 mA is drawn from the 7-V fixed voltage power supply

• Power dissipation due to output drive current (when the output on duty is 100%)

= (232/8 k) + (222/10 k) = 0.12 W

Therefore, the total power dissipation for the whole IC is:

In stop mode:

P = P2 = 0.19 W

In start mode:

P = P1 + P3 + P4 + P5 = 6.51 W

(For a output on duty of 100%)

16. IC Temperature Rise Measurement Techniques

• Thermocouple measurement

Attach the thermocouple to a fin on the heat sink when using a thermocouple to measure the IC temperature. This

technique is simple but is subject to large measurement errors when the heat generation is not consistent.

• Measurement using IC internal diode characteristics

We recommend using the parasitic diode that exists between the INT.IN and ground in this IC. (According to

Sanyo data, the temperature characteristic of this diode is about 1.8 V/°C for ID = 1 mA.) The external resistor

must be removed during testing.

No. 5439-9/10

LB1921