Datasheet
Table Of Contents
- Figure 1. Typical application circuit
- 1 Pin settings
- 2 Maximum ratings
- 3 Electrical characteristics
- 4 Functional description
- 5 Application notes - buck conversion
- 5.1 Closing the loop
- 5.2 GCO(s) control to output transfer function
- 5.3 Error amplifier compensation network
- 5.4 LED small signal model
- 5.5 Total loop gain
- 5.6 Compensation network design
- 5.7 Example of system design
- 5.8 Dimming operation
- 5.9 Component selection
- 5.10 Layout considerations
- 5.11 Thermal considerations
- 5.12 Short-circuit protection
- 5.13 Application circuit
- 6 Application notes - alternative topologies
- 7 Package mechanical data
- 8 Ordering information
- 9 Revision history
LED5000 Application notes - buck conversion
Doc ID 023951 Rev 1 23/51
the external power components and the compensation network are selected, a direct
measurement to determine T
RISE
, T
FALL
(see
Equation 24
) is necessary to certify the
achieved dimming performance.
5.9 Component selection
5.9.1 Sensing resistor
In closed loop operation the LED5000 feedback pin voltage is 200 mV, so the sensing
resistor calculation is expressed as:
Equation 26
Since the main loop (see
Chapter 5.1
) regulates the sensing resistor voltage drop, the
average current is regulated into the LEDs. The integration period is at minimum 5*T
SW
since the system bandwidth can be dimensioned up to f
SW
/5 at maximum.
A system loop based on a peak current mode architecture features consistent advantages in
comparison with simpler closed loop regulation schemes like the hysteretic or the constant
ON/OFF control.
The system performs the output current regulation over a period which is at least five times
longer than the switching frequency. The output current regulation neglects the ripple
current contribution and its reliance on external parameters like input voltage and output
voltage variations (line transient and LED forward voltage spread). This performance can
not be achieved with simpler regulation loops like hysteretic control.
For the same reason, the switching frequency is constant over the application conditions,
that helps to tune the EMI filtering and to guarantee the maximum LED current ripple
specification in the application range. This performance cannot be achieved using constant
ON/OFF time architectures.
5.9.2 Inductor and output capacitor selection
The output capacitor filters the inductor current ripple that, given the application condition,
depends on the inductor value. As a consequence the LED current ripple, that is the main
specification for a switching current source, depends on the inductor and output capacitor
selection.
Figure 16. Equivalent circuit
R
S
200 mV
I
LED
---------------------=
DCRDCR DCRDCR
COUTCOUT
RsRs
VINVIN
LL
ESRESR
Rd1Rd1
VINVIN
LL
ESRESR
Dled1Dled1
1 2
DD
COUTCOUT
1
2
DD
RsRs
RdnRdn
DlednDledn
AM13500v1