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UC3656 Schematic ( PDF Datasheet ) - Unisonic Technologies

Teilenummer UC3656
Beschreibung 800mA SYNCHRONOUS BUCK CONTROLLER
Hersteller Unisonic Technologies
Logo Unisonic Technologies Logo 




Gesamt 7 Seiten
UC3656 Datasheet, Funktion
UNISONIC TECHNOLOGIES CO., LTD
UC3656
Preliminary
1MHz,800mA SYNCHRONOUS
BUCK CONTROLLER
„ DESCRIPTION
The UTC UC3656 is a synchronous buck controller for a DC/DC
converter optimized for high-performance microprocessor applications.
It is configured with current mode for excellent line and load
transient response. At 1MHz operation frequency, the use of small
surface mount inductor and capacitor is enabled. The built-in
synchronous switch saves the use of an external Schottky diode and
increases efficiency.
The UTC UC3656 is a low-noise synchronous step-down DC/DC
converter that is ideally suited for systems requiring supplies from a
1-cell Li-ion battery or from a 3-cell to 4-cell NiCd, NiMH, or alkaline
battery.
The UTC UC3656 is generally applied in power supply for portable
devices and USB-based power system.
„ FEATURES
* High Efficiency (95%) Possible
* Input Voltage Range: 2.5V ~ 5.5V
* Schottky Diode is Saved
* Output Current: 800mA (@5V Input 3.3V Output)
* 1.0MHz Oscillation Frequency
* Internal ON/OFF
* 100% Duty Cycle Allows Low Dropout Operation
* Stand-by Current: 1μA (Max.)
„ ORDERING INFORMATION
Ordering Number
UC3656G-AF5-R
Package
SOT-25
Packing
Tape Reel
CMOS IC
„ MARKING
www.unisonic.com.tw
Copyright © 2010 Unisonic Technologies Co., Ltd
1 of 7
QW-R502-469.A
Free Datasheet http://www.Datasheet4U.com






UC3656 Datasheet, Funktion
UC3656
Preliminary
CMOS IC
„ APPLICATION INFORMATION
Capacitor Selection
Proper selection of capacitors using the UTC UC3656 plays a critical role for system performance. The source
current of the top MOSFET is square wave of duty cycle in a continuous mode.
The input capacitor is generally used to provide a low impedance loop for the edges of pulsed current drawn by
the UTC UC3656. A load step at the output can induce ringing at the input VIN. This ringing can couple to the output
and be mistaken as loop instability. Increasing the capacitance of the input capacitor can great improve the
oscillation. A typical value is 4.7~ 22μF ceramic (X5R or X7R), POSCAP or Aluminum Polymer. With these
capacitors, better high frequency bypassing is achieved; resistive losses are thus reduced by their low ESR for
higher efficiency. Moreover, the input capacitor RMS current changes with the input voltage and the output voltage.
The maximum RMS current in the input capacitor can be expressed in the following formula:
IRMS = IOMAX
VO
VIN
(1
VO
VIN
)
The output capacitor depends upon the suitable ripple voltage. Low ripple voltage corresponds to lower effective
series resistance (ESR).
The output ripple voltage can be calculated by:
ΔVOUT
ΔIL(ESR +
1
8fCOUT
)
The formula as follows shows the output capacitor RMS ripple current:
IRMS = 1
23
VOUT × (VIN VOUT )
L × f × VIN
For the UTC UC3656, X5R or X7R dielectric formulations have the priority in choosing the input and output
ceramic capacitors. This is because these dielectrics provide the best temperature and voltage characteristics of all
the ceramics for a given value and size.
Inductor Selection
Selecting an optimum inductor is dependent upon the expected ripple current and large value inductors lower
ripple current and small value inductors result in higher ripple current. In addition, the losses associated with the
DCR and its effect on the total converter efficiency should be taken consideration.
The inductor is also chosen to limit the ripple current to some pre-fixed value, approx. 20 ~ 40% (TYP.) of the
full load current at the maximum input voltage.
The inductance value can be calculated in the following equation:
ΔIL = 0.2 ~ 0.4 × IOUT(MAX)
L
=
VOUT
f × ΔIL
(1
VOUT
VIN
)
Adjustable Output Voltage
A resistive divider adjusts the output voltage of the UTC UC3656, which divides a 0.6V reference voltage at FB
pin. The output voltage can be obtained by the equation as follows:
VOUT
= 0.6V
× (1+
R1 )
R2
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
6 of 7
QW-R502-469.A
Free Datasheet http://www.Datasheet4U.com

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