TK65600B Toko America Incorporated, TK65600B Datasheet - Page 9

• Manufacturer Part Number: TK65600B
• Description: Inductive White Led Driver
• Manufacturer: Toko America Incorporated

www.DataSheet4U.com
TK65600B
Theory of Operation Cont.
A classic boost configuration is not able to provide short circuit protection, as the input voltage
source can provide current to the load, through the inductor and diode, even if the circuit is dis-
abled. A synchronous rectifier is required, in order to be able to provide short circuit protection.
The synchronous rectifier (Msr) is replacing the diode found in classic boost circuits. The main
advantage is eliminating the need for an external component. The second important advantage is
the potential for less voltage drop across this device. A serious drawback is the fact that a FET is a
non-directional device, unlike the diode it replaces, so, while the diode operated by itself, careful
control of the synchronous rectifier operation is required. The synchronous rectifier must be off , at
all times when the inductor switch is on - otherwise, shoot-through current from the boost capacitor,
through the synchronous rectifier and through the inductor switch, to the ground, can occur - this
cannot be allowed to happen, because of its effect on efficiency.
A second issue to consider when driving the synchronous rectifier is the fact that, the FET being a
non-directional device, the drive circuitry must ensure that the synchronous rectifier is on only
when the boost voltage (output) is smaller than the voltage at the inductor node - otherwise, the
boost (output) capacitor will discharge through the synchronous rectifier FET and inductor, to the
input voltage source (Vdd).
When the inductor switch is off and the synchronous rectifier it is held off because the inductor
voltage is smaller than the boost voltage, both power FET’s (Mind and Msr) are off. When this
happens, the remaining energy in the inductor may be enough to start ringing, using the inductor
and whatever parasitic capacitance can find (both Mind and Msr are large devices, with large
parasitic capacitance). The resulting oscillations can be large enough to trigger the hysteresis
comparator in the internal synchronous rectifier driver circuitry. Also, this ringing oscillation may
cause noise in other parts of the application’s system. To avoid these effects, a snubber circuit is
used, to short the inductor node not to ground (that would be a loss of energy), but back to Vdd
(charging back the source). The snubber circuit must carefully select the moment when Mind and
Msr are off, following the current ramp-up in the inductor, and NOT preceding it. The state machine
inside the snubber does that. There is another moment when both Mind and Msr are off at the
same time - when the inductor switch is cut off, after ramping the current in the inductor, but the
synchronous rectifier, Msr, is not yet on (due to delays in circuitry, etc.). At this point in time, the
inductor node voltage is highest and no snubber effect is acceptable.
July 2, 2003 TOKO, Inc.
Page 9