eup3409 Eutech Microelectronics Inc, eup3409 Datasheet - Page 8

• Manufacturer Part Number: eup3409
• Description: Dual 1.5mhz, 800ma Synchronous Step-down Converter
• Manufacturer: Eutech Microelectronics Inc

Operation
The EUP3409 has dual independent slop-compensated
constant frequency current mode PWM step-down
converters. Both the main (P-channel MOSFET) and
synchronous (N-channel MOSFET) switches are internal.
During normal operation, the EUP3409 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 the
output capacitor to GND, or through the main and
synchronous switch to GND.
Application Information
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 960mA rated
inductor should be enough for most applications
(800mA+160mA). 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 EUP3409. 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:
DS3409 Ver1.0 Jan. 2008
OUT
IN
and C
also increases the ripple current as shown in
∆I
OUT
L
L
=320mA (40% of 800mA).
=
Selection
(f)(L)
1
V
OUT




1
−
V
V
OUT
IN




OUT
/V
IN
IN
. The
or
8
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 below
figure.
I
∆V
RMS
OUT
=
V
I
≅
OUT
O
OUT
∆I
×
L
is driven by the required effective
=
OUT
V




V
ESR
0.6V
IN
O
has a strong effect on loop
×
EUP3409



+




1
1
+
8fC
−
OUT
OUT
R2
R1
V
V
OUT
1
IN
is determined by:
O



has been met, the








RIPPLE(P-P)