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PDF UC3886 Data sheet ( Hoja de datos )
| Número de pieza | UC3886 | |
| Descripción | Average Current Mode PWM Controller IC | |
| Fabricantes | Unitrode | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de UC3886 (archivo pdf) en la parte inferior de esta página. Total 8 Páginas | ||
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Average Current Mode PWM Controller IC
UC1886
UC2886
UC3886
FEATURES
• 10.3V - 20V Operating Range
• Low Offset Voltage Amplifier
• High Bandwidth Current and Voltage
Amplifiers
• Low Offset Current Sense Amplifier
• Undervoltage Lockout
• Trimmed 5 Volt Reference
• Externally Programmable Oscillator
Charge Current
• 1.5A Peak Totem Pole Output
• Available in 16-pin DIL or SOIC
Packages
BLOCK DIAGRAM
DESCRIPTION
The UC3886 family of PWM controller ICs is designed for DC-to-DC con-
verters with average current mode control. It is designed for use in con-
junction with the UC3910 4-bit DAC and Voltage Monitor. The UC3886
drives an external N-channel MOSFET and can be used to power the In-
tel Pentium® Pro and other high-end microprocessors.
The UC3886 in conjunction with the UC3910 converts 5VDC to an adjust-
able output ranging from 2.0V to 3.5V in 100mV steps with 35mV DC sys-
tem accuracy.
The oscillator is programmed by the user’s selection of an external resis-
tor and capacitor, and is designed for 300kHz typical operation.
The voltage and current amplifiers have 3.5MHz gain-bandwidth product
to satisfy high performance system requirements.
The internal current sense amplifier permits the use of a low value current
sense resistor, minimizing power loss. It has inputs and outputs accessi-
ble to allow user-selection of gain-setting resistors, and is internally com-
pensated for a gain of 5 and above. The command voltage input is
buffered and provided for use as the reference for the current sense am-
plifier.
The output of the voltage amplifier (input to the current amplifier) is
clamped to 1 volt above the command voltage to serve as a current limit.
The gate output can be disabled by bringing the CAO/ENBL pin to below
0.8 volts.
6/98
UDG-95098-2
1 page  
APPLICATION INFORMATION
OSCILLATOR
The UC3886 oscillator is a saw tooth. The rising edge
is governed by a current controlled by RT flowing into
the capacitor CT. The falling edge of the sawtooth sets
the dead time for the output. Selection of RT should be
done first, based on desired maximum duty cycle. CT
can then be chosen based on the desired frequency,
FS, and the value of RT. The design equations are:
DMAX
= 1 RT
V2. 0
mA• 4. 0
( )FOSC
V2. 0 • (4. 0mA • RT) 2. 0V
=
CT V• 1.8 • RT mA• 4.0
UC1886
UC2886
UC3886
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0
20
40
60
80
100
120
R6 (kΩ)
Figure 2. Programming Maximum Duty Cycle with RT
1000
100
100pF
220pF
470pF
UDG-96022
Figure 1. Oscillator
Configuring the Current Sense Amplifier
The UC3886 Current Sense Amplifier is used to amplify
a differential current sense signal across a low value
current sense resistor, RSENSE. This amplifier must be
set up as a differential amplifier as shown.
The Current Sense Amplifier gain, GCSA, is given by the
ratio of R2/R1. The output of the Current Sense Ampli-
fier at the ISO pin is given by
VISO
=VBUF
+ VSENSE
•
R2
R1
The Current Sense Amplifier gain, GCSA, must be pro-
grammed to be greater than or equal to 5.0 (14dB), as
this amplifier is not stable with gain below 5.0. The Cur-
rent Sense Amplifier gain is limited on the high side by
its Gain-Bandwidth product of 2.5MHz. Therefore GCSA
must be programmed between
GCSA_MIN = 5.0 and GCSA_MAX = 2.5MHz/FSWITCH
10
0
1nF
20 40 60 80 100
RT (kΩ)
Figure 3. Programming Switching Frequency with CT
0.600
0.500
0.400
RT = 5k
RT = 100k
0.300
0.200
0.100
0.000
100
300
500
700
CT (pF)
Figure 4. Deadtime vs. CT and RT
900
1100
5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet UC3886.PDF ] | |
Hoja de datos destacado
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