uc3750 Unisonic Technologies, uc3750 Datasheet - Page 7

• Manufacturer Part Number: uc3750
• Description: 600khz Pwm/pfm Step-down Dc-dc Controller
• Manufacturer: Unisonic Technologies

UC3750
8. Inductor Value Calculation
requirements into consideration. The fact is that lower inductor value can make the cost, PC board space smaller
and can provide faster transient response while resulting in higher ripple current and core losses. The loading
current, I
i.e. 0.5A x 40% = 0.2A. The relationship between the inductor value and inductor ripple current is as followed:
9. P-Channel Power MOSFET Selection
the power MOSFET are the gate threshold, V
voltage operation need a low gate threshold device that can work down to the minimum input voltage level. R
determines the conduction losses for each switching cycle, the lower the ON resistance, the higher and the efficiency
can be achieved. A power MOSFET with lower gate charge can give lower switching losses but the fast transient can
cause unwanted EMI to the system. Compromise is required during the design stage.
10. Flywheel Diode Selection
on the P-Channel switch duty cycle. At high input voltages, the diode conducts most of the time. In case of V
approaches V
diode will subject to its highest stress. Under this condition, the diode must be able to safely handle the peak current
circulating in the loop. So, it is important to select a flywheel diode that can meet the diode peak current and average
power dissipation requirements. Under normal conditions, the average current conducted by the flywheel diode is
given by:
and fast switching times.
11. Input and Output Capacitor Selection (C
(V
prevent large input voltage transients. The maximum RMS input current, can be estimated by the equation below:
worst-case condition is used for design. Selecting of the output capacitor, the required effective series resistance
(ESR) of the capacitor decide the C
is met. The output voltage ripple, V
can see from equation 4 that the output voltage ripple contributed by two parts. For a better performance, Low ESR
tantalum capacitors should be used. The major contributor is the capacitor ESR for most. Ordinary
aluminum-electrolytic capacitors have high ESR and should be avoided while higher quality Low ESR
aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Surface-mount tantalum capacitors are
better and provide neat and compact solution for space sensitive applications.
OUT
Selecting the proper inductance must take an inductor’s physical size, transient respond and power conversion
Where R
The operation of UTC UC3750 must be used with an external P-Channel power MOSFET. The main keys for
The flywheel diode is turned on and carries load current during the off time. The average diode current depends
Where I
A fast switching diode must also be used to optimize efficiency. Schottky diodes are ideal for low forward drop
In continuous mode operation, the source current of the P-Channel MOSFET is a square wave of duty cycle
I
Where F
L
I
I
V
DESCRIPTION INFORMATION(Cont.)
RMS (MAX)
D
RMS
RIPPLE
+V
=
=
(
T
OUT
MAX
V
F
ON
)/V
V
IN
IN
D
UNISONIC TECHNOLOGIES CO., LTD
)
( ×
-
= 0.5A and the inductor ripple current, I
≈
www.unisonic.com.tw
IN
DS (ON)
OSC
+
V
≈
has a maximum value at V
OUT
is the average diode current and V
. We must select a low ESR input capacitor that can support the maximum RMS input current to
I
V
OUT
V
L
I
I
L
OUT
IN
F
-
, the diode conducts only a small fraction of the cycle. While the output terminals are shorted, the
is the switching frequency and ESR is the effective series resistance of the output capacitor. We
-
R
-
RIPPLE
I ×
IPPLE
R
is the ON resistance of the external P-channel MOSFET.
×
DS
OUT
V
(
OUT
(P
ON
-
P)
)
(
P (
I ×
V
×
IN
V
OUT
-
(ESR+
IN
) P
-
V
-
OUT
V
RIPPLE
OUT
OUT
4
)
1
2
F
OSC
)
and the capacitance will be adequate for filtering unless the ESR requirement
is approximated by:
1
C
IN
OUT
= 2V
GS
IN
Preliminary
, the “ON” resistance, R
)
F
and C
OUT
is the forward diode voltage drop.
L−RIPPLE(P−P)
, where I
OUT
)
RMS (MAX)
is designed to be less than 40% of the load current,
DS (ON)
= I
OUT
and its total gate charge, Q
/2. As a general practice, this simple
QW-R502-360.a
CMOS IC
T
. Low input
7 of 9
DS (ON)
IN