eup3406 Eutech Microelectronics Inc, eup3406 Datasheet - Page 9

• Manufacturer Part Number: eup3406
• Description: 1.5mhz, 600ma Synchronous Step-down Converter
• Manufacturer: Eutech Microelectronics Inc

Application Information
Main Control Loop
The EUP3406 uses a slop-compensated constant
frequency, current mode PWM architecture. Both the
main
(N-channel MOSFET) switches are internal. During
normal operation, the EUP3406 regulates output voltage
by switching at a constant frequency and then
modulating the power transferred to the load each cycle
using PWM comparator. It sums three weighted
differential signals: the output feedback voltage from an
external resistor divider, the main switch current sense,
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 keep 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
V
equation. A reasonable starting point for setting ripple
current is ∆I
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 720mA rated
inductor should be enough for most applications
(600mA+120mA). For better efficiency, choose a low
DC-resistance inductor.
C
In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle V
primary function of the input capacitor is to provide a
low impedance loop for the edges of pulsed current
drawn by the EUP3406. 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.
DS3406 Ver1.2 Nov. 2007
∆I
OUT
IN
L
and C
=
also increases the ripple current as shown in
(f)(L)
(P-Channel
1
OUT
L
=240mA (40% of 600mA).
V
Selection
OUT




1
MOSFET)
−
V
V
OUT
IN




and
OUT
synchronous
/V
IN
IN
. The
or
9
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:
The output capacitor C
stability.
The selection of C
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
RMS current rating generally far exceeds the I
requirement. The output ripple ∆V
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:
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure3.
∆V
V
I
OUT
RMS
OUT
=
=
≅
0.6V
I
O
∆I
×
L







1
V
ESR
+
Figure3.
V
OUT
IN
O
R2
R1
+
×
is driven by the required effective





 

OUT
8fC
1
−
OUT
1
has a strong effect on loop
V
V
EUP3406
IN
O






 

OUT
OUT
is determined by:
has been met, the
RIPPLE(P-P)