NXP Semiconductors SSL21083, Руководство пользователя, Страница: 8 / 18

UM10501 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
User manual Rev. 2 — 16 November 2011 8 of 18
NXP Semiconductors
UM10501
SSL21083 reference board user manual
6. Changing the output current
The SSL21083 monitors the charging current in the inductor using the sense resistors R5
and R6. It controls a MOSFET to retain a constant peak current. In addition, the IC
supports valley switching.
These features enable a driver to operate in Boundary Conduction Mode (BCM) with
valley switching where the average current in the inductor is the output current.
The SSL21083 turns off the MOSFET when the voltage on pin SOURCE reaches 500 mV.
If the value of R5 in parallel with R6 is 2

, the peak current is limited to 250 mA.
(1)
When the MOSFET is turned off, inductor L2 is discharged and the current flowing
through
the inductor decreases. When the current in the inductor reaches 0 mA, the
voltage on the DRAIN pin starts to oscillate because of the stray capacitance (ringing).
SSL21083 waits for a valley of this oscillation.
The charge time of the inductor is calculated using Equation 2 :
(2)
The discharge time of the inductor is calculated using Equation 3 :
(3)
When the inductor is charging/discharging, a current flows through it. However, there is
also an effective current when ringing. Consider the oscillation frequency when adjusting
the output current. It is calculated using
Equation 4 :
(4)
The time from the start of oscillation to the first valley is calculated using Equation 5 :
(5)
The output current is calculated using Equation 6 . The resulting output current is:
(6)
Therefore by changing I
peak
we can change I
LED
.
I
peak
0.5 R5 R6 +
  
R5 R6

---------------------------------------
=
t
ch
L2
2 I
LED

V
i
V
LED
–
------------------------

=
t
dch
L2
2 I
LED

V
LED
--------------------

=
f
ring
1
2
 
L2 C
FET

C5
   
--------------------------------------------------------------------
=
t
ring
1
2 f
ring

-------------------
=
I
LED
1
2
---
I
peak
t
ch
t
dch
+
t
ch
t
dch
t
ring
+ +
--------------------------------------
 
=