EUP3410/3411
Functional Description
current (the load current plus half the peak-to-peak
inductor ripple current) is below the 2.4A minimum peak
current limit.
The EUP3410/3411 is a current-mode step-down
switching regulator. The device regulates an output
voltage as low as 1.2V from a 4.5V to 16V input power
supply. The device can provide up to 2Amp continuous
current to the output. The EUP3410/3411 uses
current-mode architecture to control the regulator loop.
The output voltage is measured at FB through a resistive
voltage divider and amplified through the internal error
amplifier. The output current of the transconductance
error amplifier is presented at COMP pin where a RC
network compensates the regulator loop. Slope
compensation is internally added to eliminate
subharmonic oscillation at high duty cycle. The slope
compensation adds voltage ramp to the inductor current
signal which reduces maximum inductor peak current at
high duty cycles.
The device uses an internal Hside n-channel switch to
step down the input voltage to the regulated output
voltage. Since the Hside n-channel switch requires gate
voltage greater than the input voltage, a boostrap BS
capacitor is connected between SW and BS to drive the
n-channel gate. The BS capacitor is internally charged
while the switch is off. An internal 6.8ꢀ switch from SW
to GND is added to insure that SW is pulled to GND
when the switch is off to fully charge the BS capacitor.
The inductance value can be calculated by the equation:
L =
VOUT
∗
VIN − VOUT / VIN ∗f ∗∆I
Where VOUT is the output voltage, VIN is the input
voltage, f is the switching frequency, and ∆I is the
peak-to-peak inductor ripple current.
Input Capacitor
The input current to the step-down converter is
discontinuous, and therefore an input capacitor C1 is
required to supply the AC current to the step-down
converter while maintaining the DC input voltage. A low
ESR capacitor is required to keep the noise minimum at
the IC. Ceramic capacitors are preferred, but tantalum or
low-ESR electrolytic capacitors may also suffice. The
input capacitor value should be greater than 10µF, and
the RMS current rating should be greater than
approximately 1/2 of the DC load current. In Figure 2,
for insuring stable operation C2 should be placed as
close to the IC as possible. Alternately a smaller high
quality ceramic 0.1µF capacitor may be placed closer to
the IC and a larger capacitor placed further away. If
using this technique, it is recommended that the larger
capacitor type are either tantalum or electrolytic. In
Figure 1, all ceramic capacitors should be placed close
to the EUP3410/3411.
Application Information
Setting the Output Voltage
The output voltage is set through a resistive voltage
divider (see Figure1 or 2). The voltage divider divides
the output voltage down by the ratio:
Output Capacitor
The output capacitor is required to maintain the DC
output voltage. Low ESR capacitors are preferred to
keep the output voltage ripple low. The characteristics of
the output capacitor also affect the stability of the
regulator control loop. Ceramic, tantalum, or low ESR
electrolyticcapacitors are recommended. In the case
of ceramic capacitors, the impedance at the switching
frequency is dominated by the capacitance. The output
voltage ripple is estimated to be:
VFB = VOUT ∗ R3 / R2 + R3 = 1.2V
( )
Thus the output voltage is :
VOUT = 1.2V ∗ R2 + R3 / R3
( )
Choose R3 value in the range 10k to 100k, R2 is
determined by :
VRIPPLE ~=1.4∗ VIN ∗ fLC / f
( )
∧ 2
R2 = VOUT /1.2 −1 ∗ R3
( )
Where VRIPPLE is the output ripple voltage, VIN is the
input voltage, fLC is the resonant frequency of the LC
filter, f is the switching frequency. In the case of
tanatalum or low ESR electrolytic capacitors, the ESR
dominates the impedance at the switching frequency, and
so the output ripple is calculated as:
For example, for a 3.3V output voltage, R3 is 10Kꢀ, and
R2 is 17.5Kꢀ.
Inductor
The inductor is required to supply constant current to the
output load while being driven by the switched input
voltage. A larger value inductor results in less ripple
current and lower output ripple voltage. However, the
larger value inductor has a larger physical size, higher
series resistance, and lower saturation current. Choose
an inductor that does not saturate under the worst-case
load conditions. A good rule for determining the
inductance is to allow the peak-to- peak ripple current in
the inductor to be approximately 30% of the maximum
load current. Also, make sure that the peak inductor
VRIPPLE ~= ∆I∗ R ESR
Where VRIPPLE is the output voltage ripple, ∆I is the
inductor ripple current, and RESR is the equivalent series
resistance of the output capacitors.
DS3410/3411 Ver1.2 Nov. 2008
9
EUTECH [ EUTECH MICROELECTRONICS INC ]
