L6730B datasheet(31/52 Pages) STMICROELECTRONICS

31 / 52 page

L6730 - L6730B

Device description

Doc ID 11938 Rev 3

31/52

5.14.1

Fan power supply failure

In many applications the fan is driven by a DC motor that uses a DC/DC converter. Often

only the speed of the motor is controlled by varying the voltage applied to the input terminal

and there is no control on the torque because the current is not directly controlled. The

current has to be limited in case of overload or short-circuit, but without stopping the motor.

With the L6730B, the current can be limited without shutting down the system because

constant current protection is provided. In order to vary the motor speed, the output voltage

of the converter must be varied. Both L6730 and L6730B have a dedicated EAREF pin (see

Figure 4) which provides an external reference to the non-inverting input of the error-

amplifier.

In these applications the duty cycle depends on the motor’s speed and sometimes a 100%

duty cycle setting has to be used to attain the maximum speed. In these conditions, the

bootstrap capacitor can not be recharged and the system cannot work properly. Some PWM

controllers limit the maximum duty cycle to 80 or 90% in order to keep the bootstrap

capacitor charged, but this makes performance during the load transient worse. The

“bootstrap anti-discharging system” allows the L6730x to work at 100% without any

problem.

Figure 26.: 100% duty cycle operation on page 31 shows the following picture illustrates the

device behavior when the input voltage is 5 V and a 100% duty cycle is set by an external

reference.

5.14.2

No-sink at zero current operation

The L6730 can work in no-sink mode. If output current is zero the converter skip some

pulses and works with a lower switching frequency. Between two pulses can pass a

relatively long time (say 200-300 µs) during which there’s no switching activity and the

current into the inductor is zero. In this condition the phase node is at the output voltage and

in some cases this is not enough to keep the bootstrap cap charged. For example, if Vout is

3.3 V the voltage across the bootstrap cap is only 1.7 V. The high-side MOSFET cannot be

Figure 26.

100% duty cycle operation

Vout=5V

Vin=5V

LGate

Fsw?6.3KHz

TOFF

≈200ns

≈

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