EUP3408
Application Information
Main Control Loop
The EUP3408 uses a slope-compensated constant
frequency, current mode architecture. Both the main
(P-Channel MOSFET) and synchronous (N-channel
MOSFET) switches are internal. During normal
operation, the EUP3408 regulates output voltage by
switching at a constant frequency and then modulating
the power transferred to the load each cycle using PWM
comparator. The duty cycle is controlled by three
weighted differential signals: the output of error
amplifier, the main switch sense voltage and the
slope-compensation ramp. It modulates output power by
adjusting the inductor-peak current during the first half
of each cycle. An N-channel, synchronous switch turns
on during the second half of each cycle (off time). When
the inductor current starts to reverse or when the PWM
reaches the end of the oscillator period, the synchronous
switch turns off. This keeps excess current from flowing
backward through the inductor, from the output
capacitor to GND, or through the main and synchronous
switch to GND.
Inductor Selection
The output inductor is selected to limit the ripple current
to some predetermined value, typically 20%~40% of the
full load current at the maximum input voltage. Large
value inductors lower ripple currents. Higher V
IN
or
V
OUT
also increases the ripple current as shown in
equation. A reasonable starting point for setting ripple
current is
∆I
L
=320mA (40% of 800mA).
I
RMS
=
I
O
×
V
V
O
×
1
−
O
V
V
IN
IN
The output capacitor C
OUT
has a strong effect on loop
stability.
The selection of C
OUT
is driven by the required effective
series resistance (ESR).
ESR is a direct function of the volume of the capacitor;
that is, physically larger capacitors have lower ESR.
Once the ESR requirement for C
OUT
has been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement. The output ripple
∆V
OUT
is determined by:
1
ESR
+
∆V
≅
∆I
L
OUT
8fC
OUT
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage
characteristics of all the ceramics for a given value and
size.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
VOUT
=
0.6V
1
+
V
1
−
OUT
∆I
=
V
L (f)(L) OUT
V
IN
1
R2
R1
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 960mA rated
inductor should be enough for most applications
(800mA+160mA). For better efficiency, choose a low
DC-resistance inductor.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle V
OUT
/V
IN
. The
primary function of the input capacitor is to provide a
low impedance loop for the edges of pulsed current
drawn by the EUP3408. A low ESR input capacitor sized
for the maximum RMS current must be used. The size
required will vary depending on the load, output voltage
and input voltage source impedance characteristics. A
typical value is around 4.7µF.
The input capacitor RMS current varies with the input
voltage and the output voltage. The equation for the
maximum RMS current in the input capacitor is:
DS3408 Ver1.2
Nov. 2007
For adjustable voltage package, the external resistive
divider is connected to the output, allowing remote
voltage sensing as shown in below figure.
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
