l TEXAS
INSTRUMENTS
3
SW
M
IN OUT SW
117 10 L f
f
V V 3 f L
´ ´ ´
=
- + ´ ´
( ) ( )
( ) ( ) ( ) ( )
6
M SENSE
C2O 6
M M V OUT
167 10 f R Gi s
G s
1 17 10 f Gi s He s f G Gi s Z s
-
-
´ ´ ´ ´
=
+ ´ ´ ´ ´ - ´ ´ ´
F
LIMIT
DYNAMIC ESR
OUT
V
I
L
R R
C
D
D =
+ +
TPS92510
SLUSAE4A –JANUARY 2012–REVISED AUGUST 2012
www.ti.com
Equation 10 shows how to reduce the peak transient current (ΔI) by adding a series inductance (LLIMIT).
Typically, the bypass capacitor (COUT) is used to reduce the high-frequency ripple current through the LED.
Adding inductance into this path is counter productive. Alternatively, the buck inductance, or switching frequency
can be increased to achieve a lower LED ripple current. In this way, the output capacitor can be reduced in
value, which allows LLIMIT to be smaller. The end result of low LED ripple current can still be achieved. Reducing
the output capacitor (COUT) does not reduce the magnitude of the transient current, but it does reduce the settling
time.
(10)
Open Current Sense Resistor Fault Protection
An open current sense resistor is unlikely, but may be caused by a failing resistor, an open solder joint, or an
open PCB trace. As the current sense resistor opens, the output voltage quickly rises due to the energy stored in
the inductor, similar to an open LED fault. Therefore, it is necessary to clamp the output voltage, either with a
TVS to GND or a diode from VOUT to VIN. It is also necessary to use a current limiting resistor on the VSENSE
pin. Otherwise, the output voltage spike causes the VSENSE voltage to break down its internal ESD structure.
The ESD device should be limited to 20 mA. In this condition, the converter attempts to regulate the output
current through the ESD structure, which results in a very low regulated current. Typically, the output voltage
overcharges and holds the VSENSE voltage high within a few switching cycles. The device stops switching, and
there is a long off-time between consecutive start-up attempts. The consecutive attempts result in lower peak
current through the ESD structure than the initial event.
Shorted Output Fault
A shorted output fault is considered rare, because the system does not require any single component from VOUT
to GND. Therefore, a single component failure cannot cause this fault condition. If this failure mode were to occur
it would most likely be due to a mechanical short, or a foreign object. In the unlikely event of this failure mode,
the TPS92510 responds in the following way.
With the output voltage shorted directly to ground the COMP voltage saturates high, because the feedback
voltage is zero. The controller would naturally command wide duty cycle PH pulses. However, as soon as the
gate driver turns on, very quickly an overcurrent event is detected and the PH pulse is truncated. This results in
minimum on-time pulses at the PH node. Depending upon the impedance of the short, large currents can build
up in the inductor, which naturally raises the junction temperature of the TPS92510. The over-temperature
protection feature protects the device.
Control-to-Output Transfer Function
The TPS92510 converter uses peak current mode control in order to regulate the average LED current. Slope
compensation is utilized internally to eliminate sub-harmonic oscillations over a wide range of operating duty
cycles. To properly compensate the closed-loop system the control to output gain characteristics must be
calculated. The control to output transfer function is shown in Equation 11.
where
• RSENSE is the LED current sense resistance (11)
where
• L is the output inductance
• fSW is the switching frequency in Hz (12)
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Product Folder Links: TPS92510