Preliminary
Operation Modes
The EUP2520 has two operating modes; Figure 3 shows
main display in PWM current mode operation, the
appropriate selection of R
FB
resistor in series with four
white LEDs set the output current driving the main display.
Figure 4 shows Sub display in PFM mode, the appropriate
selection of R
SUB1
and R
SUB2
resistors set the output
voltage driving the OLED subdisplay.
EUP2520
Circuit Description
The EUP2520 is designed for White LED & OLED
backlighting in mobile phone applications. It has a main
display loop which can drive up to 5 white LEDS in
series and a sub display loop which is designed to drive
OLED up to 20V/50 mA. The main display loop
employs a fixed frequency current mode scheme to
regulate the LED current. The sub display loop employs
a fixed frequency gated oscillator scheme to regulate the
output voltage. The device has two independent control
pins to enable the Main or Sub displays. Note that both
displays can not be ON at the same time
PWM Operation
The EUP2520 utilizes a synchronous Current Mode PWM
control scheme to regulate the feedback voltage over all
load and line conditions for the main display. The
EUP2520 is internally compensated preventing the need
for external compensation components yielding a compact
solution. The operation can best be understood referring
to the functional block diagram. The EUP2520 operates as
follows: During the first cycle, the oscillator sets the
driver logic and turns on the NMOS power device
conducting current through the inductor and reverse
biases the external diode isolating the output from the
V
SW
node.
The LED current is supplied by the output capacitor when
the NMOS power device is active. During this cycle, the
output voltage of the EAMP controls the current through
the inductor. This voltage will increase for larger loads
and decrease for smaller loads limiting the peak current
in the inductor. The sum of the EAMP voltage and voltage
ramp is compared with the sensed switch voltage. Once
these voltages are equal, the PWM COMP will then reset
the logic turning off the NMOS power device and forward
biasing the external diode to the white LED load and
flows through the diode to the white LED load and output
capacitor. The inductor current recharges the output
capacitor and supplies the current for the white LED
branches. The oscillator then sets the driver logic again
repeating the process.
PFM Operation
The EUP2520 utilizes a gated oscillator control scheme
for the sub-display. There is a hysteresis window to
regulate the output voltage. The oscillator frequency is
the same as the frequency in PWM control. The Duty
cycle of the oscillator signal is always set to maximum.
During the first part of each switching cycle, the
internal NMOS switch is turned on until the PFM
current limit is reached. When the NMOS is off, the
voltage of the inductor reverses and forces current
through the diode to the output capacitor. This process
continues until the upper comparator hysteresis is
reached at which point the NMOS is disabled until the
lower comparator threshold is reached and the process
repeats again.
Figure 3. Main Display
I
LED
=
(
V
MAIN _ FB
−
V
MAIN _ RTN
)
R
FB
Figure 4. Sub Display
V
SUB
=
(
R
SUB1
+
R
SUB2
)
R
SUB2
∗
V
SUB _ FB
9
DS2520 Ver0.1 May. 2007
EUTECH [ EUTECH MICROELECTRONICS INC ]
