EUP2794
LED Headroom Voltage (VHR)
Output Current Capability
Four current sources are connected internally between
POUT and D1-D4. The voltage across each current
source, (VPOUT - VDX), is referred to as headroom
voltage (VHR). The current sources require a sufficient
amount of headroom voltage to be present across
them in order to regulate properly. Minimum required
headroom voltage is proportional to the current
flowing through the current source, as dictated by the
equation:
The primary constraint on the total current capability
is the headroom voltage requirement of the internal
current sources. Combining the VPOUT and VHR
equations from the previous two sections yields the
basic inequality for determining the validity of an
EUP2794 LED-drive application:
V
POUT
=1.5× V − I × R
IN TOTAL
OUT
V
= k
× I
HR−MIN
− V
HR DX
V
-
= k
*I
HR DX
HR M
V
≥ V
IN
POUT
DX
HR−MIN
× R
1.5× V − I
− V
OUT DX
≥
(K
× I )
HR DX
IN TOTAL
The parameter kHR, typically 20mV/mA in the
EUP2794, is a proportionality constant that represents
the ON-resistance of the internal current mirror
transistors. For worst-case design calculations, using
a kHR of 25mV/Ma is recommended. (Worst-case
recommendation accounts for parameter shifts from
part-to-part variation and applies over the full
operating temperature range). Figure 4 shows how
output current of the EUP2794 varies with respect to
headroom voltage.
Rearranging this inequality shows the estimated total
output current capability of an application:
1.5× V
)
− V
DX−MAX
−
IN−MIN
× I
I
≤
÷ R
OUT
TOTAL
(K
)
HR DX
Examining the equation above, the primary limiting
factors on total output current capability are input and
LED forward voltage. A low input voltage combined
with a high LED voltage may result in insufficient
headroom voltage across the current sources, causing
them to fall out of regulation. When the current
sources are not regulated, LED currents will be below
desired levels and brightness matching will be highly
dependent on LED forward voltage matching.
Typical EUP2794 output resistance is 3.0Ω. For
worst-case design calculations, using an output
resistance of 3.5Ω is recommended . EUP2794 has a
typical kHR constant of 20mV/mA. For worst-case
design calculations, use kHR =25mV/mA. (Worst-case
recommendations account for parameter shifts from
part-to-part variation and apply over the full operating
temperature range). ROUT and kHR increase slightly
with temperature, but losses are typically offset by the
negative temperature coefficient properties of LED
forward voltages. Power dissipation and internal
self-heating may also limit output current capability
but is discussed in a later section.
Figure 4. ILED VS VHR 4 LEDs, VIN=3.0V
On the flat part of the graph, the currents regulate
properly as there is sufficient headroom voltage for
regulation. On the sloping part of the graph the
headroom voltage is too small, the current sources are
squeezed, and their current drive capability is limited
Changes in headroom voltage from one output to the
next, possible with LED forward voltage mismatch,
will result in different output currents and LED
brightness mismatch. Thus, operating the EUP2794
with insufficient headroom voltage across the current
sources should be avoided.
DS2794 Ver0.5 Mar. 2006
10
EUTECH [ EUTECH MICROELECTRONICS INC ]
