CS3845B Datasheet PDF - Cherry Semiconductor Corporation
| Part Number | CS3845B | |
| Description | Current Mode PWM Control Circuit with 50% Max Duty Cycle | |
| Manufacturers | Cherry Semiconductor Corporation | |
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CS3845B
Current Mode PWM Control Circuit
with 50% Max Duty Cycle
Description
Features
The CS3845B provides all the neces-
sary features to implement off-line
fixed frequency current-mode control
with a minimum number of external
components.
The CS3845B incorporates a precision
temperature-controlled oscillator to
minimize variations in frequency. An
internal toggle flip-flop, which blanks
the output off every other clock cycle,
ensures that the duty-cycle never
exceeds 50%. The undervoltage lock-
out feature ensures that VREF is stabi-
lized within specification before the
output stage is enabled. The CS3845B
has been optimized for lower start up
current (500µA max).
Other features include 1% trimmed
band gap reference, pulse-by-pulse
current limiting, and a high-current
totem pole output for driving capaci-
tive loads.
Absolute Maximum Ratings
Supply Voltage (ICC<30mA) ..........................................................Self Limiting
Supply Voltage (Low Impedance Source)...................................................30V
Output Current ...............................................................................................±1A
Output Energy (Capacitive Load) .................................................................5µJ
Analog Inputs (VFB, VSENSE)...........................................................-0.3V to 5.5V
Error Amp Output Sink Current...............................................................10mA
Lead Temperature Soldering
Wave Solder (through hole styles only) ..........10 sec. max, 260°C peak
Reflow (SMD styles only) ...........60 sec. max above 183°C, 230°C peak
Block Diagram
s Optimized for Off-line Use
s Temperature
Compensated Oscillator
s 50% Maximum Duty-cycle
Clamp
s Low Start-up Current
(500µA max)
s Pulse-by-pulse Current
Limiting
s Undervoltage Active Pull
Down
s Double Pulse Suppression
s 1% Trimmed Bandgap
Reference
s High Current Totem Pole
Output
Package Options
8 Lead PDIP & SO
VCC
Gnd
VFB
COMP
OSC
Sense
VCC Undervoltage Lock-out
34V
8.4V/7.6V
Error
Amplifier
-
+
2.50V
Oscillator
Set/ 5.0 Volt
Reset Reference
R
VREF
Undervoltage
Lockout
R
Toggle
Flip-Flop
NOR
2R
R
S
R
1V
Current PWM
Sensing Latch
Comparator
Internal
Bias
VCC Pwr
VREF
VOUT
Pwr Gnd
COMP 1
VFB 2
Sense 3
OSC 4
8 VREF
7 VCC
6 VOUT
5 Gnd
14 Lead SO Narrow
COMP 1
NC 2
VFB 3
NC 4
Sense 5
NC 6
OSC 7
14 VREF
13 NC
12 VCC
11 VCC Pwr
10 VOUT
9 Gnd
8 Pwr Gnd
Rev. 11/19/98
Cherry Semiconductor Corporation
2000 South County Trail, East Greenwich, RI 02818
Tel: (401)885-3600 Fax: (401)885-5786
Email: [email protected]
Web Site: www.cherry-semi.com
1 A ® Company
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VOSC
OSC
RESET
Toggle
F/F Output
EA Output
Switch
Current
VCC
IO
VO
Figure 2: Timing Diagram
Vupper
Vlower
ton
tC
ton = tC
toff = tC+2td
Figure 3: Timing Parameters.
Circuit Description: continued
Setting the Oscillator
The parameters Tc and Td can be determined as follows:
( )tc = RTCT ln
VREF - Vlower
VREF - Vupper
( )td = RTCT ln
VREF - IdRT - Vlower
VREF - IdRT - Vupper
Substituting in typical values for the parameters in the
above formulas:
VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA,
then
tc ≈ 0.5534RTCT
( )td = RTCT ln
2.3 - 0.0083 RT
4.0 - 0.0083 RT
For better accuracy RT should be ≥10kΩ.
Grounding
toff
High peak currents associated with capacitive loads neces-
sitate careful grounding techniques. Timing and bypass
td capacitors should be connected close to Gnd in a single
point ground.
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
5
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| Information | Total 6 Pages | |
| Link URL | [ Copy URL to Clipboard ] | |
| Download | [ CS3845B.PDF Datasheet ] | |
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