TK65600B Toko America Incorporated, TK65600B Datasheet - Page 8

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

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The TK65600 is an inductive white LED driver circuit. The input voltage is 2.7V up to 6V. The
load is represented by white LED’s - one or more parallel strings of LED’s, each string consisting
of two or more LED’s connected in series. The absolute maximum voltage allowed at the output
pin is 16V, dictated by wafer process limits. The forward drop voltage of the LED’s dictates how
many LED’s can be in a string, as the voltage at the output pin is the voltage across the LED’s in
series in a string, plus the voltage drop across the feedback resistor. The feedback resistor ap-
pears in series with the load, connected between the bottom terminal of the LED string(s) and
ground.
The minimum input voltage of 2.7V and the maximum output voltage of about 15V requires this
circuit to be a boost circuit - TK65600 is an inductive boost circuit. The circuit regulates the current
in the load, as the light intensity of the LED’s depends on the current flowing through them. The
LED current information is provided by a feedback resistor, connected between the load and
ground. A classical current -mode control loop, using pulse width modulation (PWM) at a fixed
frequency, regulates the boost circuit output, such as to maintain the current in the LED’s constant.
As with any classical current -mode control loop PWM, the boost converter has the feature of pulse
by pulse current limiting. On the TK65600 that current loop limit is set to about 400mA.
Therefore, the inductor, which is to be used with the TK65600, should have an Isat above 400mA.
There are a few additional functions the circuit incorporates:
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TK65600B
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Disable - allows the circuit to be turned on and off by an external enable signal (off for
V
Over-Voltage Protection (OVP) - shuts off the power FET’s if the output voltage rises above a
predetermined threshold (14V). This is intended to prevent damage to the circuit for an open
load condition, for instance, by not allowing the output voltage to rise above the preset limit.
Short-Circuit Protection (SCP) - if the output sees an unusually high load or a short-circuit, there
is circuitry provided that will cut off the current path to the output, wait a predetermined amount
of time, then attempt to restart. If the output short-circuit or heavy loading condition at the output
disappeared, the circuit will start and function normally. If the short-circuit condition persists,
the circuit will wait again the predetermined amount of time, then it will attempt to restart again.
The high load or short-circuit condition is identified, for the purpose of this feature, by a low
output voltage (less than 1.2V). In order to provide for start-up condition (when the output
voltage is inherently low), the SCP circuitry waits for a little while, before asserting the short
circuit condition signal. That little while is set now at sixteen (16) clock cycles, while the reset
time, that is, the time before the circuit attempts to restart, it is set now at (512) clock cycles.
With a clock of 600kHz, these times are approximately 27us for asserting the short circuit condi-
tion signal and about 853us between attempts to restart the boost circuit.
enable
<0.3V, on for V
enable
>1.2V)
Theory of Operation
July 2, 2003 TOKO, Inc.