Electronic Components Datasheet Search
  English  ▼


UC3845BDR2G Datasheet(PDF) 7 Page - ON Semiconductor

Part # UC3845BDR2G
Description  High Performance Current Mode Controllers
Download  19 Pages
Scroll/Zoom Zoom In 100%  Zoom Out
Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

UC3845BDR2G Datasheet(HTML) 7 Page - ON Semiconductor

Back Button UC3845BDR2G Datasheet HTML 3Page - ON Semiconductor UC3845BDR2G Datasheet HTML 4Page - ON Semiconductor UC3845BDR2G Datasheet HTML 5Page - ON Semiconductor UC3845BDR2G Datasheet HTML 6Page - ON Semiconductor UC3845BDR2G Datasheet HTML 7Page - ON Semiconductor UC3845BDR2G Datasheet HTML 8Page - ON Semiconductor UC3845BDR2G Datasheet HTML 9Page - ON Semiconductor UC3845BDR2G Datasheet HTML 10Page - ON Semiconductor UC3845BDR2G Datasheet HTML 11Page - ON Semiconductor Next Button
Zoom Inzoom in Zoom Outzoom out
 7 / 19 page
background image
UC3844B, UC3845B, UC2844B, UC2845B
The UC3844B, UC3845B series are high performance,
fixed frequency, current mode controllers. They are
specifically designed for Off−Line and DC−DC converter
applications offering the designer a cost−effective solution
with minimal external components. A representative block
diagram is shown in Figure 16.
The oscillator frequency is programmed by the values
selected for the timing components RT and CT. Capacitor CT
is charged from the 5.0 V reference through resistor RT to
approximately 2.8 V and discharged to 1.2 V by an internal
current sink. During the discharge of CT, the oscillator
generates an internal blanking pulse that holds the center
input of the NOR gate high. This causes the Output to be in
a low state, thus producing a controlled amount of output
deadtime. An internal flip−flop has been incorporated in the
UCX844/5B which blanks the output off every other clock
cycle by holding one of the inputs of the NOR gate high. This
in combination with the CT discharge period yields output
deadtimes programmable from 50% to 70%. Figure 2 shows
RT versus Oscillator Frequency and Figure 3, Output
Deadtime versus Frequency, both for given values of CT.
Note that many values of RT and CT will give the same
oscillator frequency but only one combination will yield a
specific output deadtime at a given frequency. The oscillator
thresholds are temperature compensated to within
at 50 kHz. Also, because of industry trends moving the
UC384X into higher and higher frequency applications, the
UC384XB is guaranteed to within
±10% at 250 kHz.
In many noise−sensitive applications it may be desirable
to frequency−lock the converter to an external system clock.
This can be accomplished by applying a clock signal to the
circuit shown in Figure 18. For reliable locking, the
free−running oscillator frequency should be set about 10%
less than the clock frequency. A method for multi−unit
synchronization is shown in Figure 19. By tailoring the
clock waveform, accurate Output duty cycle clamping can
be achieved to realize output deadtimes of greater than 70%.
Error Amplifier
A fully compensated Error Amplifier with access to the
inverting input and output is provided. It features a typical
dc voltage gain of 90 dB, and a unity gain bandwidth of
1.0 MHz with 57 degrees of phase margin (Figure 6). The
non−inverting input is internally biased at 2.5 V and is not
pinned out. The converter output voltage is typically divided
down and monitored by the inverting input. The maximum
input bias current is −2.0
mA which can cause an output
voltage error that is equal to the product of the input bias
current and the equivalent input divider source resistance.
The Error Amp Output (Pin 1) is provided for external
loop compensation (Figure 29). The output voltage is offset
by two diode drops (
≈1.4 V) and divided by three before it
connects to the inverting input of the Current Sense
Comparator. This guarantees that no drive pulses appear at
the Output (Pin 6) when Pin 1 is at its lowest state (VOL).
This occurs when the power supply is operating and the load
is removed, or at the beginning of a soft−start interval
(Figures 21, 22). The Error Amp minimum feedback
resistance is limited by the amplifier’s source current
(0.5 mA) and the required output voltage (VOH) to reach the
comparator’s 1.0 V clamp level:
3.0 (1.0 V) + 1.4 V
0.5 mA
= 8800 W
Current Sense Comparator and PWM Latch
The UC3844B, UC3845B operate as a current mode
controller, whereby output switch conduction is initiated by
the oscillator and terminated when the peak inductor current
reaches the threshold level established by the Error
Amplifier Output/Compensation (Pin 1). Thus the error
signal controls the peak inductor current on a
cycle−by−cycle basis. The Current Sense Comparator PWM
Latch configuration used ensures that only a single pulse
appears at the Output during any given oscillator cycle. The
inductor current is converted to a voltage by inserting the
ground−referenced sense resistor RS in series with the
source of output switch Q1. This voltage is monitored by the
Current Sense Input (Pin 3) and compared to a level derived
from the Error Amp Output. The peak inductor current under
normal operating conditions is controlled by the voltage at
Pin 1 where:
Ipk =
V(Pin 1) − 1.4 V
3 RS
Abnormal operating conditions occur when the power
supply output is overloaded or if output voltage sensing is
lost. Under these conditions, the Current Sense Comparator
threshold will be internally clamped to 1.0 V. Therefore the
maximum peak switch current is:
Ipk(max) =
1.0 V
When designing a high power switching regulator it
becomes desirable to reduce the internal clamp voltage in order
to keep the power dissipation of RS to a reasonable level. A
simple method to adjust this voltage is shown in Figure 20. The
two external diodes are used to compensate the internal diodes,
yielding a constant clamp voltage over temperature. Erratic
operation due to noise pickup can result if there is an excessive
reduction of the Ipk(max) clamp voltage.
A narrow spike on the leading edge of the current
waveform can usually be observed and may cause the power
supply to exhibit an instability when the output is lightly
loaded. This spike is due to the power transformer
interwinding capacitance and output rectifier recovery time.
The addition of an RC filter on the Current Sense Input with
a time constant that approximates the spike duration will
usually eliminate the instability (refer to Figure 24).

Similar Part No. - UC3845BDR2G

ManufacturerPart #DatasheetDescription
ON Semiconductor
UC3845BDR2 ONSEMI-UC3845BDR2 Datasheet
433Kb / 20P
September, 2004 ??Rev. 3
More results

Similar Description - UC3845BDR2G

ManufacturerPart #DatasheetDescription
Motorola, Inc
UC3842B MOTOROLA-UC3842B Datasheet
401Kb / 16P
Freescale Semiconductor...
MC34129EF FREESCALE-MC34129EF Datasheet
444Kb / 16P
   high performance current mode controllers
ON Semiconductor
UC3844BNG ONSEMI-UC3844BNG Datasheet
399Kb / 19P
   High Performance Current Mode Controllers
August, 2013 ??Rev. 11
UC3845BVDG ONSEMI-UC3845BVDG Datasheet
399Kb / 19P
   High Performance Current Mode Controllers
August, 2013 ??Rev. 11
UC3845BVDR2G ONSEMI-UC3845BVDR2G Datasheet
399Kb / 19P
   High Performance Current Mode Controllers
August, 2013 ??Rev. 11
UC3844BVDG ONSEMI-UC3844BVDG Datasheet
399Kb / 19P
   High Performance Current Mode Controllers
August, 2013 ??Rev. 11
Unisonic Technologies
UC3842B UTC-UC3842B_08 Datasheet
518Kb / 11P
Motorola, Inc
UC3844 MOTOROLA-UC3844 Datasheet
376Kb / 14P
ON Semiconductor
UC3842A ONSEMI-UC3842A Datasheet
399Kb / 14P
1996 REV 1
UC3844 ONSEMI-UC3844 Datasheet
376Kb / 14P
1996 REV 1
More results

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Go To PDF Page

Link URL

Privacy Policy
Does ALLDATASHEET help your business so far?  [ DONATE ] 

About Alldatasheet   |   Advertisement   |   Datasheet Upload   |   Contact us   |   Privacy Policy   |   Link Exchange   |   Manufacturer List
All Rights Reserved©Alldatasheet.com

Mirror Sites
English : Alldatasheet.com  |   English : Alldatasheet.net  |   Chinese : Alldatasheetcn.com  |   German : Alldatasheetde.com  |   Japanese : Alldatasheet.jp
Russian : Alldatasheetru.com  |   Korean : Alldatasheet.co.kr  |   Spanish : Alldatasheet.es  |   French : Alldatasheet.fr  |   Italian : Alldatasheetit.com
Portuguese : Alldatasheetpt.com  |   Polish : Alldatasheet.pl  |   Vietnamese : Alldatasheet.vn
Indian : Alldatasheet.in  |   Mexican : Alldatasheet.com.mx  |   British : Alldatasheet.co.uk  |   New Zealand : Alldatasheet.co.nz
Family Site : ic2ic.com  |   icmetro.com