CS3845B
CS3845B Current Mode PWM Control Circuit with 50% Max Duty Cycle
Description
The CS3845B provides all the necessary 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 lockout feature ensures that VREF is stabilized 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 capacitive loads.
Features
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
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
Package Options
8 Lead PDIP & SO
V CC
V CC Undervoltage Lock-out
V CC Pwr
COMP VFB
1 2 3 4
8 7 6 5
VREF VCC VOUT Gnd
34V Gnd 8.4V/7.6V V FB Error Amplifier + OSC 2.50V
Set/ 5.0 Volt Reset Reference Internal Bias R V REF Undervoltage Lockout Toggle Flip-Flop
V REF
Sense OSC
14 Lead SO Narrow
COMP 1 NC 2
NOR V OUT
14 13 12 11 10 9 8
COMP
VREF NC VCC VCC Pwr VOUT Gnd Pwr Gnd
R
Oscillator
VFB 3
2 R R Sense 1V Current PWM Sensing Latch Comparator S Pwr Gnd R
NC 4 Sense 5 NC 6 OSC 7
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
Rev. 11/19/98
1
A
®
Company
CS3845B
Electrical Characteristics: 0≤TA≤70˚C, VCC=15V; RT=10kΩ, CT=3.3nF for sawtooth mode, unless otherwise stated
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
s Reference Section Output Voltage Line Regulation Load Regulation Temperature Stability Total Output Variation Output Noise Voltage Long Term Stability Output Short Circuit s Oscillator.