NCS29001
http://onsemi.com
11
VREF REFERENCE VOLTAGE
The device contains an accurate 5 V reference that can supply up to 10 mA and can be accessed through the VREF pin. It
can be used to program the LED feedback voltage by using a resistor divider on the FBP pin. This reference is only active when
STBY = low. When the device is in standby mode the VREF pin voltage will drop to 4.2 V typical with a minimum of 3.5 V.
The VREF will return to 5 V immediately when STBY is driven high.
MINIMUM ON & OFF TIME
If the steady state duty cycle and switching frequency combine to generate short Ton times (low VOUT/VIN converter ratio),
the converter will skip some cycles to regulate V
OUT
which will increase output voltage ripple. The timing limit is set by the
intrinsic loop propagation delay and the switching frequency will be limited by the minimum ON time and OFF time.
THE INDUCTOR SELECTION
For a given application, it is necessary to know the input voltage at the inductor (VIN
INDUCTOR
), the output current (I
OUT
)
set by RFBN and the voltage on the FBP pin, and the switching frequency (F
sw
). The inductor can be chosen using the formula
below:
L
max
t
1
2 F
sw
I
OUT
ǒ
V
IN
V
OUT
Ǔ
2
ǒ
V
OUT
* V
IN
Ǔ
(eq. 1)
The minimal inductor value is determined with the desired peak current flowing through the inductor. Using the chosen
inductor value the steady state duty cycle and peak inductor current can be calculated:
D +
2 L F
sw
I
OUT
ǒ
V
OUT
* V
IN
Ǔ
Ǹ
V
IN
(eq. 2)
And the inductor peak current is now:
I
peak
+
V
IN
D
L F
sw
+
2 I
OUT
(V
OUT
* V
IN
)
L F
sw
Ǹ
(eq. 3)
THE CURRENT SENSE RESISTOR
Set a current limit between 2 and 2.5 times the peak inductor current to account for inductor tolerance:
I
limit
+ 2.5 I
peak
(eq. 4)
The current limit reference fixed on the over-current protection comparator is V
CS
= 0.5 V and the resistance can be calculated
using following the equation:
R
CS
+
V
CS
2.5 I
peak
(eq. 5)
SLOPE COMPENSATION
After the current sense resistor is calculated additional calculations are needed for the external slope compensation ramp. Using
the R
SENSE
value the typical slope of the compensation ramp can be calculated:
Mramp +
1
2
R
SENSE
V
OUT
* V
IN
L
(eq. 6)
Using the typical value for , the external compensation resistor can be calculated as follows:
R
SC
+
M
RAMP
I
RAMP
(eq. 7)
The slope compensation ramp has an offset current, , which is used to calculate the peak ramp current and finally the adjusted
current sense resistor.
I
RAMP,peak
+ I
OFF
) D
I
RAMP
R
SW
(eq. 8)
R
CS
+
V
CS
* R
CS
I
RAMP,peak
I
limit
) I
RAMP,peak
(eq. 9)
