EUP3484S
Functional Description
voltage. However, the larger value inductor will have a
larger physical size, higher series resistance, and/or
lower saturation current. A good rule for determining
inductance is to allow the peak-to-peak ripple current
to be approximately 30% of the maximum switch
current limit. Also, make sure that the peak inductor
current is below the maximum switch current limit.
The EUP3484S regulates input voltages from 4.5V to
24V down to an output voltage as low as 0.925V, and
supplies up to 3A of load current.
The EUP3484S uses current-mode control to regulate
the output voltage. The output voltage is measured at
FB through a resistive voltage divider and amplified
through the internal transconductance error amplifier.
The voltage at the COMP pin is compared to the switch
current (measured internally) to control the output
voltage.
The converter uses internal N-Channel MOSFET
switches to step-down the input voltage to the regulated
output voltage. Since the high side MOSFET requires a
gate voltage greater than the input voltage, a boost
capacitor connected between SW and BS is needed to
drive the high side gate. The boost capacitor is charged
from the internal 5V rail when SW is low.
The inductance value can be calculated by:
V
V
OUT
∆I
OUT
L =
1 −
f
V
S
L
IN
Where VOUT is the output voltage, VIN is the input
voltage, fS is the switching frequency, and ∆IL is the
peak-to-peak inductor ripple current.
Choose an inductor that will not saturate under the
maximum inductor peak current, calculated by:
At light loads, the inductor current may reach zero or
reverse on each pulse. The bottom DMOS is turned off
by the current reversal comparator and the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator. At
light load, the EUP3484S will automatically skip
pulses in pulse skipping mode operation to maintain
output regulation and increases efficiency.
When the FB pin voltage exceeds 15% of the nominal
regulation value of 0.925V, the over voltage
comparator is tripped and forcing the high-side switch
off.
V
V
OUT
2 f
OUT
I
= I
+
1 −
LP
LOAD
L
V
S
IN
Where ILOAD is the load current.
The choice of which style inductor to use mainly
depends on the price vs. size requirements and any
EMI constraints.
Optional Schottky Diode
During the transition between the high-side switch and
low-side switch, the body diode of the low-side power
MOSFET conducts the inductor current. The forward
voltage of this body diode may be high and cause
efficiency loss. An optional small 1A Schottky diode
B130 in parallel with low-side switch is recommended
to improve overall efficiency when input voltage is
higher.
Application Information
Setting the Output Voltage
The output voltage is set using a resistive voltage
divider connected from the output voltage to FB. The
voltage divider divides the output voltage down to the
Input Capacitor
feedback voltage by the ratio:
R2
The input current to the step-down converter is
discontinuous, therefore a capacitor is required to
supply the AC current while maintaining the DC input
voltage. Use low ESR capacitors for the best
performance. Ceramic capacitors are preferred, but
tantalum or low-ESR electrolytic capacitors will also
suffice. Choose X5R or X7R dielectrics when using
ceramic capacitors.
Since the input capacitor (C1) absorbs the input
switching current, it requires an adequate ripple current
rating. The RMS current in the input capacitor can be
estimated by:
V
= V
FB
OUT
R1 + R2
Thus the output voltage is:
R1 + R2
V
= 0.925
OUT
R2
R2 can be as high as 100kꢀ, but a typical value is
10kꢀ. Using the typical value for R2, R1 is determined
by:
kΩ
R1 = 10.81
V
− 0.925
OUT
For example, for a 3.3V output voltage, R2 is 10kꢀ
and R1 is 26.1kꢀ.
V
V
OUT
OUT
I
= I
1−
C1
LOAD
V
V
Inductor
IN
IN
The inductor is required to supply constant current to
the load while being driven by the switched input
voltage. A larger value inductor will result in less ripple
current that will in turn result in lower output ripple
The worst-case condition occurs at VIN = 2VOUT, where
IC1 = ILOAD/2. For simplification, use an input capacitor
with a RMS current rating greater than half of the
maximum load current.
DS3484S Ver1.2 May 2012
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
