AT9917 Datasheet, PDF (8/12 Pages) Supertex, Inc – Automotive LED Driver IC with High Current Accuracy

AT9917
(minimum), allowing the IREF pin to be pulled all the way to
0V without triggering the short circuit comparator. Therefore,
the minimum voltage for a short circuit detection is 250mV.
PWM Dimming (PWMD)
PWM dimming in the AT9917 can be accomplished using a
TTL-compatible square wave source at the PWMD pin.
Fault Conditions
The AT9917 is a robust controller which can protect the LEDs
and the LED driver in case of fault conditions. The AT9917
includes both open LED protection and output short circuit
protection. In both cases, the AT9917 shuts down and at-
tempts a restart. The hiccup time is programmed by the ca-
pacitor at the JTR pin.
When the PWM signal is high, the GATE and FLT pins are _{When a fault condition is detected, both GATE and FLT out-}
enabled and the output of the transconductance op-amp _{puts are disabled, the COMP pins and JTR pins are pulled to}
is connected to the external compensation network. Thus, _{GND. Once the voltage at the JTR pin falls below 0.1V and}
the internal amplifier controls the output current. When the _{the fault condition(s) have disappeared, the capacitor at the}
PWMD signal goes low, the output of the transconductance _{JTR pin is released and is charged slowly by a 10μA current}
amplifier is disconnected from the compensation network. _{source. Once the capacitor is charged to 0.7V, the COMP}
Thus, the integrating capacitor maintains the voltage across _{pins are released and GATE and FLT pins are allowed to turn}
it. The GATE is disabled, so the converter stops switching _{on. If the hiccup time is long enough, it will ensure that the}
and the FLT pin goes low, turning off the disconnect switch. _{compensation networks are all completely discharged and}
that the converters start at minimum duty cycle.
Note that disconnecting the LED load during PWM dimming
causes the energy stored in the inductor to be dumped into _{Short Circuit Protection}
the output capacitor. The filter capacitor that is chosen should _{When a short circuit condition is detected (output current be-}
be large enough so that it can absorb the inductor energy _{comes higher than twice the steady state current), the GATE}
without significant change of the voltage across it. If the ca- _{and FLT outputs are pulled low. As soon as the disconnect}
pacitor voltage change is significant, it would cause a turn-on _{FET is turned off, the output current goes to zero and the}
spike in the inductor current when PWMD goes high.
short circuit condition disappears. At this time, the hiccup
Jitter and Hiccup Timer (JTR)
The JTR pin is a multipurpose pin in the AT9917. It is used
to set the jitter frequency (frequency at which the switching
frequency swings between its limits). It is also used to set the
hiccup time for fault conditions.
timer is started. Once the timing is complete, the converter
attempts to restart. If the fault condition still persists, the con-
verter shuts down and goes through the cycle again. If the
fault condition is cleared (due to a momentary output short)
the converter will start regulating the output current normally.
This allows the LED driver to recover from accidental shorts
without having to reset the IC.
The value of the capacitor required for the jitter frequency is
given by:
During short circuit conditions, there are two conditions that
5.0µF
C_JTR = _FJTR(Hz)
determine the hiccup time.
The first is the time required to discharge the compensation
capacitors. Assuming a pole-zero R-C network at the COMP
Note that the jitter frequency must be chosen to be signifi- _{pin (series combination of RZ and CZ in parallel with CC),}
cantly lower than the cross over frequency of the closed loop
control. If not, the controller will not be able to reject the jitter
frequency and the LED current will have a current ripple at
the jitter frequency.
t_COMP = 3 • R_Z • C_Z
In case the compensation networks are only type 1 (single
capacitor), then:
The same capacitor is used to determine the hiccup time.
The hiccup time is computed as:
t_COMP = 3 • 300Ω • C_C
t_HICCUP
=
C_JTR • 0.6V
10µA
The second is the time required for the inductors to com-
pletely discharge following a short circuit. This time can be
computed as:
If the hiccup time is lower than desired, the capacitor at the
pin can be increased at the cost of a lower jitter frequency.
π
t_IND = ₄ √L • C_O
Supertex inc. ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: [phone] ● www.supertex.com
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