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Feed forward compensation for ZXSC300 LED driver
Yong Ang, Application Engineer, Zetex Semiconductors
Input voltage feed forward compensation for ZXSC300 to improve control of the
LED current
Introduction
The ZXSC300 LED drivers do not directly control the LED current. As a consequence the LED
current is dependent of the input voltage. This application note describes a way of reducing the
supply voltage dependency by a method of supply voltage feed forward compensation. The
method can also be used to provide temperature compensation of the LED.
The ZXSC300 works on the PFM control scheme where the LED current is simply regulated by
controlling the peak current through transistor Q1. The internal voltage threshold of current sense
pin is around 19mV and transistor Q1 is switched off when its current reaches the preset
threshold, thereby necessitating fewer external components required. However, this threshold
value is invariant to the supply voltage level. In the event where input voltage increases, peak Q1
current will stay the same and current delivered to the LED creeps up which could potentially
damage the LED if it exceeds the maximum rated current of the device.
The circuit diagram in Figure 1 shows how to apply input voltage and thermal correction to a
typical LED. A simple design guide for a single LED driver has also been put forward. The
equations can generate a design capable of sourcing up to 200mA LED current, when used with
the Zetex high current gain NPN transistor-ZXTN25012EFH.
Input voltage feed forward compensation
Normally, IPK is set by the output current threshold voltage VISENSE divided by RSENSE. As the
input voltage increases, the inductor ripple current level ⌬I decreases because the transistor off
time, TOFF is fixed by the ZXSC300
⌬I = (VOUT - VIN) • TOFF ÷ L
L discharges at a flatter slope to a higher minimum choke current IMIN = IPK - ⌬I, before transistor
Q1 is turned on again.
Figure 1
Circuit diagram of ZXSC300 with feed forward and thermal compensation
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Consequently, the average current IAV flowing through L increases and a shorter transistor on
time, TON is required to charge boost inductor to the preset threshold current level IPK
TON = ⌬I • L ÷ VIN
By making the aforementioned assumptions for turn-on period and average coil current, the
output power delivered to the LED is now determined from
POUT = VLED • IAV • TOFF ÷ (TON + TOFF)
Therefore, a higher power and LED current is delivered to the LED at high VIN for a fixed RSENSE
and this elevated current could potentially damage the LED if it exceeds the maximum rated
current of the device.
Ignoring the effect of thermistor RT for the moment, a 100⍀ resistor ROFF can be inserted in series
with RSENSE and feed forward resistor Rfb (see Figure 1) to inject a slight voltage offset across
resistor RSENSE. This enables a lower Q1's current to build up the required VISENSE to turn the
driver off, which regulates the LED current. The Rfb value has to be sufficiently big to lower
dissipation and to prevent circuit from stalling. The circuit could stall at high input voltage if Rfb
drops 19mV or more across 100⍀ resistor forcing the driver off all the time.
It must be noted that ISENSE pin threshold on ZXSC300 has a positive temperature coefficient of
0.4%/°C. If a circuit nominal operating temperature is higher than 65°C, it could give
approximately 20% increase in average LED current from that in 25°C ambient. When a feed
forward network is used, this injects an offset voltage to the threshold pin. For instance, if an
offset voltage of 9.5mV is used, the effective VISENSE temperature coefficient becomes double.
Therefore, it is essential that thermal compensation is used with a feed forward approach.
Feed forward components calculation
For initial estimation, the associated IAV(VMAX) that delivers the required LED current can be
determined from
IAV(VMAX) = POUT ÷ (F • TOFF • VOUT)
Where the transistor switching frequency F is given by
F = VIN(MAX) ÷ VOUT ÷ TOFF
Figure 2
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Example of current and voltage waveforms for circuit using ZXSC300
with feed forward network
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IAV(VMAX) is used to establish the required DC current rating, IDC for boost inductor L.
The minimum inductor current is given by,
IMIN(VMAX) = IAVE – 0.5 • (VOUT – VIN(MAX)) • TOFF /L
A high L value is recommended to minimize errors due to propagation delays at high input
voltage, which results in increased ripple and lower efficiency.
And the maximum inductor current which relates to the Q1 peak current is
IPK(VMAX) = 2 • IAVE - IMIN(VMAX)
In practice, a higher IPK(VMAX) value can be used to account for the VCE saturation and switching
edge loss in the transistor.
The value of feed forward resistor Rfb is selected to give IPK(VMIN) at worse case input voltage and
IPK(VMAX) at maximum input voltage. The internal VISENSE threshold on the ZXSC300 is typically
19mV with ±25% tolerance at 25°C. RSENSE has to drop less voltage than that demanded by
VISENSE as Rfb will make a contribution to satisfy the threshold, which lowers IPK value with
increasing input. Allowing for the positive temperature coefficient on ISENSE pin, effective
threshold voltage level at operating temperature TAMB is;
[email protected] = 19mV ± 25% • 0.4%/°C • (TAMB - 25°C).
At low supply voltage VIN(MIN)
[email protected] = IPK(VMIN) • RSENSE + VIN(MIN) • 100⍀ ÷ (Rfb + 100⍀)
Whilst at VIN(MAX),
[email protected] = IPK(VMAX) • RSENSE + VIN(MAX) • 100⍀ ÷ (Rfb + 100⍀)
Solving the above simultaneous equations gives the required RSENSE and Rfb resistor values.
These design equations are also available as a spreadsheet calculator from Zetex website at
www.zetex.com/zxsc300feedforward
Figure 3 shows the measured LED current against variation in the input voltage with feed forward
compensation. For comparison purpose, the same measurement is repeated with feed forward
network removed, in which case the LED current at low supply is 3 times lower than that at
nominal input voltage level.
70
LED Current (A)
60
50
Feed forward
compensation
No feed forward
network
40
30
20
10
0
1.6 1.8
2
2.2 2.4 2.6 2.8
3
Input Voltage (V)
Figure 3
LED current discrepancy for ZXSC300 with feed forward compensation
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The improvement in LED current regulation through feed forward compensation is self-evident.
Although some discrepancy in LED current persists at low supply, this is predominantly due to
the dependency of internal VISENSE threshold level on the input voltage level.
To incorporate thermal compensation into the design, Rfb can be made up from a series
combination of normal resistor R1 and NTC RT. During start-up condition, the printed circuit
board’s and LED’s temperatures are low, hence RT has high resistance. As circuit temperature
rises to its design operating value, the effective feed forward resistance drops, increasing the
offset voltage on ISENSE pin, which in turn matches the elevated VISENSE value and hence
regulates the actual output current fed to the LED.
For instance, the required effective feed forward resistor value (R1+ RT) for 25°C ambient start-up
can be determined from
Rfb = VIN(MAX) • 100⍀ • (19mV ± 25% - IPK(VMAX) • RSENSE)
And the required normal resistor R1 is equivalent to Rfb - RT.
For this design, three NTC values (3.3K⍀, 4.7K⍀ and 6.8K⍀) are recommended. These resistors
with MURATA 0603 or 0805 size NTC thermistors with beta-constant value of 3950K are chosen to
give good current control response at both normal operating temperature and start-up
conditions. The NTC works to reduce the peak transistor current, facilitating thermal feedback
control to ensure that LED current and lumen maintenance expectation are achieved. Note that it
is sometimes difficult to achieve perfect LED current matching between start-up and normal
operating temperature. In extreme cases of large temperature gradients, the average LED current
should be lower at start-up giving less lumen output, and then ramps up to the rated current once
it reaches the normal operating temperature. Furthermore, the thermistor can be thermally
coupled to the LED to provide response tracking and prevent overheating.
Conclusion
Two or three additional external components can be used to provide input voltage feed forward
for ZXSC300. This serves to ensure that the LED current is closely regulated. The LED current
regulation improves significantly when feed forward compensation is employed. The LED current
at the worse case input voltage increases from 33% to 64% of the nominal LED current with a feed
forward network. The remaining discrepancy is predominantly due to the dependency of the
VISENSE threshold level on the input voltage level.
In applications where the circuit is designed to operate in elevated ambient temperature, a NTC
thermistor can be incorporated to facilitate thermal feedback control and prevent over heating.
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