l6730cqtr STMicroelectronics, l6730cqtr Datasheet - Page 28

• Manufacturer Part Number: l6730cqtr
• Description: Adjustable Step-down Controller With Synchronous Rectification
• Manufacturer: STMicroelectronics

Device description
5.14
5.14.1 Fan’s power supply
Figure 23. 100% duty cycle operation
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T
T
Bootstrap anti-discharging system
This built-in system avoids that the voltage across the bootstrap capacitor becomes less than
3.3V. An internal comparator senses the voltage across the external bootstrap capacitor
keeping it charged, eventually turning-on the low-side MOSFET for approximately 200ns. If the
bootstrap capacitor is not enough charged the high-side MOSFET cannot be effectively turned-
on and it will present a higher R
possible to mention at least two application conditions during which the bootstrap capacitor can
be discharged:
In many applications the FAN is a DC MOTOR driven by a voltage-mode DC/DC converter.
Often only the speed of the MOTOR is controlled by varying the voltage applied to the input
terminal and there’s no control on the torque because the current is not directly controlled.
Obviously the current has to be limited in case of overload or short-circuit but without stopping
the MOTOR. With the L6730D the current can be limited without shutting down the system
because a constant-current-protection is provided. In order to vary the MOTOR speed the
output voltage of the converter must be varied. Both L6730C and L6730D have a dedicated pin
called EAREF (see the related section) that allows providing 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 100% has
to be set in order to go at the maximum speed. Unfortunately in these conditions the bootstrap
capacitor can not be recharged and the system cannot work properly. Some PWM controller
limits the maximum duty-cycle to 80-90% in order to keep the bootstrap cap charged but this
make worse the performance during the load transient. Thanks to the “bootstrap anti-
discharging system” the L6730X can work at 100% without any problem. The following picture
shows the device behaviour when input voltage is 5V and 100% is set by the external
reference.
OFF
OFF
200ns
200ns
DS(on)
. In some cases the OCP can be also triggered. It’s
Vout=5V
Vout=5V
LGate
F
Vin=5V
Vin=5V
SW
LGate
LGate
Fsw? 6.3KHz
Fsw? 6.3KHz
V
V
OUT
IN
= 5V
L6730C - L6730D
= 5V
6.3KHz