EUP2794
Connecting outputs in parallel does not affect internal
operation of the EUP2794 and has no impact on the
Electrical Characteristics and limits previously
presented. The available diode output current,
maximum diode voltage, and all other specifications
provided in the Electrical Characteristics table apply
to parallel output configurations, just as they do to the
standard 4-LED application circuit.
Parallel Dx Outputs for Increased Current Drive
Outputs D1 through D4 may be connected together in
any combination to drive higher currents through
fewer LEDs. For example in Figure 5, outputs D1 and
D2 are connected together to drive one LED while D3
and D4 are connected together to drive a second LED.
Thermal Protection
When the junction temperature exceeds 150°C (typ.),
the EUP2794 internal thermal protection circuitry
disables the part. This feature protects the device
from damage due to excessive power dissipation. The
device will recover and operate normally when the
junction temperature falls below 140°C (typ.). It is
important to have good thermal conduction with a
proper layout to reduce thermal resistance.
Power Efficiency
Figure 5. Two Parallel Connected LEDs
Figure 7 shows the efficiency of the EUP2794. The
change in efficiency shown by the graph comes from
the transition from Pass Mode to a gain of 1.5.
With this configuration, two parallel current sources
of equal value provide current to each LED. RSET and
VBRGT should therefore be chosen so that the current
through each output is programmed to 50% of the
desired current through the parallel connected LEDs.
For example, if 30mA is the desired drive current for
2 parallel connected LEDs , RSET and VBRGT should
be selected so that the current through each of the
outputs is 15mA. Other combinations of parallel
outputs may be implemented in similar fashions, such
as in Figure 6.
Efficiency (E) of the EUP2794 is defined here as the
ratio of the power consumed by LEDs (PLED) to the
power drawn from the input source (PIN). In the
equations below, IQ is the quiescent current of the
EUP2794, ILED is the current flowing through one
LED, VLED is the forward voltage at that LED current,
and N is the number of LEDs connected to the
regulated current outputs. In the input power
calculation, the 1.5 represents the switched capacitor
gain configuration of the EUP2794.
P
= N × V
× I
LED
= V × I
LED LED
P
IN
IN IN
= V 1.5× N × I
P
×
+ I
Q
IN
IN
LED
E =
÷ P )
IN
LED
Efficiency, as defined here, is in part dependent on
LED voltage. Variation in LED voltage does not
affect power consumed by the circuit and typically
does not relate to the brightness of the LED. For an
advanced analysis, it is recommended that power
consumed by the circuit (VIN x IIN) be evaluated
rather than power efficiency. Figure 8 shows the
power consumption of the EUP2794 Typical
Application Circuit.
Figure 6. One Parallel Connected LED
DS2794 Ver0.5 Mar. 2006
11
EUTECH [ EUTECH MICROELECTRONICS INC ]
