EUP3420
Application Information
The EUP3420 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 EUP3420 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.
Soft-Start
The EUP3420 has an internal soft-start circuit that limits
the inrush current and output voltage overshoot during
startup. The soft-start is implemented with a digital
circuit increasing the switch current in steps.
Short-Circuit Protection
As soon as the output voltage drops below 50% of the
nominal output voltage, the converter switching
frequency as well as the current limit is reduced to 50%
of the nominal value.
Input Undervoltage Lockout
The undervoltage lockout circuit prevents device
misoperation at low input voltages. It prevents the
converter from turning on the switch or rectifier
MOSFET with undefined conditions.
Inductor Selection
The EUP3420 typically uses a 2.2uH output inductor.
Larger or smaller inductor values can be used to
optimize the performance of the device for specific
operation conditions.
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
=800mA (40% of 2A).
I
=
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 2400mA rated
inductor should be enough for most applications
(2A+400mA).
The DC-resistance of the inductor directely influences
the efficiency of the converter. Therefore 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 EUP3420. 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 22µF.
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:
RMS
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
∆V
OUT
≅
∆I
L
ESR
+
8fCOUT
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.
VOUT
∆I
L
=
VOUT
1
−
VIN
(f)(L)
1
DS3420
Ver1.2
Mar. 2009
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
