EUP6514
The AC impedance of output capacitor at operating
flows mainly through output capacitor. The output
ripple voltage is described as:
Below Figures say those effects. And test conditions
are Rocset=15kΩ (over-current trip point=20.6A),
Low-side MOSFET is IR3707.
2. VIN_POR
∆VOUT = ∆VOR + ∆VOC + VOC
(2)
(3)
UGATE will continuously generate a 10kHz clock
with 1% duty cycle before VIN is recognized ready by
detecting VOPS crossing 1.5V four times (rising &
falling). ROCSET must be kept lower than 37.5kΩ for
large ROCSET will keep VOPS always higher than 1.5V.
Figure 6 shows the detail actions of OCP and POR. It
is highly recommend-ed that ROCSET be lower than
30kΩ.
∆VOUT = ∆IL × rc +
t2ic dt
1
CO
∫
t1
1 VOUT
8 COL
∆VOUT = ∆IL × ∆IL × rc +
(
1 − D
)
TS2
(4)
Where △VOR is by ESR and ΔVOC by capacitance.
For electrolytic capacitor application, typically 90 to
95% of the output voltage ripple is contributed by the
ESR of output capacitor. So Equation (4) could be
simplified as :
∆VOUT = ∆IL × rc
(5)
Users could connect capacitors in parallel to get
calculated ESR.
Output Capacitor
The selection of input capacitor is mainly based on its
maximum ripple current capability. The buck converter
draws pulsewise current from the input capacitor
during the on time of S1 as shown in Figure 3. The
RMS value of ripple current flowing through the input
capacitor is described as:
Irms = IOUT
D
(
1− D) (
A
)
(6)
Figure 5. UV_FB trigger VIN power sensing
3. Shutdown
The input capacitor must be cable of handling this
ripple current. Sometime, for higher efficiency the low
ESR capacitor is necessarily.
OPS (Over Current Setting, VIN_POR and
Shutdown)
Pulling low the OPS pin by a small single transistor
can shutdown the EUP6514 PWM controller as
shown in typical application circuit.
Soft Start
1.OCP
Sense the low-side MOSFET’s RDS(ON) to set
over-current trip point.
Connecting a resistor (ROCSET) from this pin to the
source of the upper MOSFET and the drain of the
lower MOSFET sets the over-current trip point. ROCSET
an internal 40µA current source, and the lower
MOSFET on resistance, RDS(ON), set the converter
over-current trip point (IOCSET) according to the
following equation :
A built-in soft-start is used to prevent surge current
from power supply input during power on. The
soft-start voltage is controlled by an internal digital
counter. It clamps the ramping of reference voltage at
the input of error amplifier and the pulse-width of the
output driver slowly. The typical soft-start duration is
3ms.
,
40µA × ROCSET − 0.4V
DS(ON ) of the lower MOSFET
IOCSET =
R
OPS pin function is similar to RC charging or
charging or discharging circuit, so the over-current
trip point is very sensitive to parasitic capacitance (ex.
Shut-down MOSFET) and the duty ratio.
DS6514 Ver 1.1 Aug. 2008
10
EUTECH [ EUTECH MICROELECTRONICS INC ]
