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CS2841B
CS2841B
Automotive Current Mode PWM Control Circuit
Description
The CS2841B provides all the necessary features to implement offline fixed frequency current-mode control with a minimum number of external components. The CS2841B (a variation of the CS-2843A) is designed specifically for use in automotive operation. The low start threshold voltage of 8.0V (typ), and the ability to survive 40V automotive load dump transients are important for automotive subsystem designs. The CS-2841 series has a history of quality and reliability in automotive applications. The CS2841B incorporates a precision temperature-controlled oscillator with an internally trimmed discharge current to minimize variations in frequency. Duty-cycles greater than 50% are also possible. On board logic ensures that VREF is stabilized before the output stage is enabled. Ion implant resistors provide tighter control of undervoltage lockout.
Features
s Optimized for Off-line Control s Internally Trimmed Temperature Compensated Oscillator s Maximum Duty-cycle Clamp s VREF Stabilized before Output Stage Enabled s Low Start-up Current s Pulse-by-pulse Current Limiting s Improved Undervoltage Lockout s Double Pulse Suppression s 1% Trimmed Bandgap Reference s High Current Totem Pole Output
Absolute Maximum Ratings Supply Voltage (Low Impedance Source)...................................................40V Output Current ...............................................................................................1A Output Energy (Capacitive Load) .................................................................5J Analog Inputs (VFB, Sense) ............................................................-0.3V to 5.5V Error Amp Output Sink Current...............................................................10mA Lead Temperature Soldering Wave Solder (through hole styles only) ..........10 sec. max, 260C peak Reflow (SMD styles only) ...........60 sec. max above 183C, 230C peak Block Diagram
V CC Undervoltage Lock-out Circuit Set/ 5V Reset Reference 8.0V/7.4V 2.50V OSC Internal Bias Output Enable NOR V OUT + V FB Error Amplifier COMP Sense - VC R 1V 2R S R Current Sensing Comparator PWM Latch Pwr Gnd V REF
Package Options
8 Lead PDIP
V CC Pwr
COMP VFB Sense OSC
1 2 3 4
8 7 6 5
VREF VCC VOUT Gnd
Gnd
14 Lead SO Narrow
COMP 1 NC 2 VFB 3 NC 4 Sense 5 NC 6 OSC 7
14 13 12 11 10 9 8
VREF NC VCC VCC Pwr VOUT Pwr Gnd Gnd
Oscillator
Cherry Semiconductor Corporation 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885-3600 Fax: (401)885-5786 Email: info@cherry-semi.com Web Site: www.cherry-semi.com
Rev. 6/23/99
1
A
(R)
Company
CS2841B
Electrical Characteristics: d -40TA85C RT=680k, CT=0.022F for triangular mode, VCC=15V (Note 1), RT=10k, CT=3.3nF for sawtooth mode (See Fig. 3), unless otherwise stated. PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
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 Section Initial Accuracy Sawtooth Mode: (See Fig. 3)TJ=25C Sawtooth Mode: -40CTA+85 Triangular Mode (See Fig. 3) TJ=25C 8.4VVcc16V Sawtooth Mode TMINTATMAX Triangular Mode TMINTA TMAX (Note 2) VOSC (peak to peak) TJ=25C TMINTATMAX 7.4 7.2 47 44 44 52 52 52 0.2 5 8 1.7 8.3 9.2 9.4 57 60 60 1.0 kHz kHz kHz % % % V mA mA TJ=25C, IOUT=1mA 8.4VCC16V 1IOUT20mA (Note 2) Line, Load, Temp. (Note 2) 10Hzf10kHz, TJ=25C (Note 2) TA=125C, 1000 Hrs. (Note 2) TA=25C -30 4.82 50 5 -100 25 -180 4.90 5.00 6 6 0.2 5.10 20 25 0.4 5.18 V mV mV mV/C V V mV mA
Voltage Stability Temperature Stability
Amplitude Discharge current
s Error Amp Section Input Voltage Input Bias Current AVOL Unity Gain Bandwidth PSRR Output Sink Current Output Source Current VOUT High VOUT Low s Current Sense Section Gain Maximum Input Signal PSRR Input Bias Current Delay to Output
Notes:
VCOMP=2.5V VFB=0V 2VOUT4V (Note 2) 8.4VVCC16V VFB=2.7V, VCOMP=1.1V VFB=2.3V, VCOMP=5V VFB=2.3V, RL=15k to ground VFB=2.7V, RL=15k to VREF
2.42 65 0.7 60 2 -0.5 5
2.50 -0.3 90 1.0 70 6 -0.8 6 0.7
2.58 -2.0
V A dB MHz dB mA mA V
1.1
V
(Notes 3 & 4) VCOMP=5V (Note 3) 12VVCC25V (Note 3) VSense=0V TJ=25C (Note 2)
2.85 0.9
3.00 1.0 70 -2 150
3.15 1.1 -10 300
V/V V dB A ns
1. Adjust Vcc above the start threshold before setting at 15V. 2.These parameters, although guaranteed, are not 100% tested in production.
3. Parameter measured at trip point of latch with VFB=0. 4. Gain defined as: A= VCOMP VSense ; 0 VSense 0.8V.
2
CS2841B
Electrical Characteristics: continued
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
s Output Section Output Low Level Output High Level Rise Time Fall Time Output Leakage s Total Standby Current Start-Up Current Operating Supply Current ICC VFB=VSense=0V, RT=10k, CT=3.3nF s Under-Voltage Lockout Section Start Threshold Min. Operating Voltage After Turn On 7.6 7.0 8.0 7.4 8.4 7.8 V V 0.5 11 1.0 17 mA mA ISINK=20mA ISINK=200mA ISOURCE=20mA ISOURCE=200mA TJ=25C, CL=1nF (Note 2) TJ=25C, CL=1nF (Note 2) Undervoltage Active, VOUT=0 13.0 12.0 0.1 1.5 13.5 13.5 50 50 -0.01 150 150 -10.00 0.4 2.2 V V V V ns ns A
Package Pin Description
PACKAGE PIN # PIN SYMBOL FUNCTION
8L PDIP 1 2 3 4 5
14L SO Narrow 1 3 5 7 8 9 COMP VFB Sense OSC Gnd Pwr Gnd VOUT VCCPwr VCC VREF NC Error amp output, used to compensate error amplifier Error amp inverting input Noninverting input to Current Sense Comparator Oscillator timing network with Capacitor to Ground, resistor to VREF Ground Output driver Ground Output drive pin Output driver positive supply Positive power supply Output of 5V internal reference No Connection
6
10 11
7 8
12 14 2,4,6,13
3
CS2841B
Typical Performance Characteristics:
Oscillator Frequency vs CT
100 900 90 800 80 DUTY CYCLE (%) 70 60 50 40 30 20
Oscillator Duty Cycle vs RT
RT =680
700 FREQ. (kHz) 600 500
RT =1.5k
400 300 200 100
RT =10k
10
.0005
.001
.002
.003
.005
.01
.02
.03 .04 .05
100
200
300 400 500 700
1k
2k
3k 4k 5k
7k
10k
CT (F)
RT ()
Test Circuit
V REF RT
2N2222 100k 4.7k 1k Error Amp Adjust 4.7k 5k
A COMP V REF
0.1F
V CC
V FB
V CC
0.1F 1k 1W
Sense Adjust
Sense
V OUT
VO
OSC
Gnd
Gnd CT
Circuit Description
Undervoltage Lockout
V CC
ON/OFF Command to reset of IC
During Undervoltage Lockout (Figure 1), the output driver is biased to a high impedance state. The output should be shunted to ground with a resistor to prevent output leakage current from activating the power switch.
V ON = 8.0V V OFF = 7.4V
PWM Waveform
To generate the PWM waveform, the control voltage from the error amplifier is compared to a current sense signal which represents the peak output inductor current (Figure 2). An increase in VCC causes the inductor current slope to increase, thus reducing the duty cycle. This is an inherent feed-forward characteristic of current mode control, since the control voltage does not have to change during changes of input supply voltage. When the power supply sees a sudden large output current increase, the control voltage will increase allowing the duty cycle to momentarily increase. Since the duty cycle tends to exceed the maximum allowed to prevent trans4
I CC
<15mA <1mA 7.4V 8.0V
Figure 1: Typical Undervoltage Characteristics
V CC
CS2841B
Circuit Description: continued former saturation in some power supplies, the internal oscillator waveform provides the maximum duty cycle clamp as programmed by the selection of OSC components.
OSC OSC RESET EA Output Switch Current VCC
Setting the Oscillator
Oscillator timing capacitor, CT, is charged by VREF through RT and discharged by an internal current source. During the discharge time, the internal clock signal blanks out the output to the Low state, thus providing a user selected maximum duty cycle clamp. Charge and discharge times are determined by the general formulas:
IO
t c = RTCT ln
VO
(
VREF - Vlower VREF - Vupper
) )
Figure 2: Timing Diagram for key CS2841B parameters
t d = RTCT ln
(
VREF - IdRT - Vlower VREF - IdRT - Vupper
V REF RT OSC CT Gnd
Substituting in typical values for the parameters in the above formulas: VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA tc 0.5534RTCT td = RTCT ln
(
2.3 - 0.0083 RT 4.0 - 0.0083 RT
)
Timing parameters
Vupper
The frequency and maximum duty cycle can be determined from the Typical Performance Characteristic graphs.
Grounding
Vlower tc
Sawtooth Mode
td
LARGE RT (10k) VOSC
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected close to Gnd pin in a single point ground. The transistor and 5k potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Sense.
Internal Clock
Triangular Mode
SMALL RT (700k) VOSC
Internal Clock
Figure 3: Oscillator Timing Network and parameters
5
CS2841B
Package Specification
PACKAGE DIMENSIONS IN mm (INCHES) PACKAGE THERMAL DATA
D Lead Count 8 Lead PDIP 14 Lead SO Narrow Metric Max Min 10.16 9.02 8.75 8.55 English Max Min .400 .355 .344 .337
Thermal Data RJC RJA typ typ
8L PDIP 52 100
14 L SO Narrow 30 125
C/W C/W
Surface Mount Narrow Body (D); 150 mil wide
4.00 (.157) 3.80 (.150)
6.20 (.244) 5.80 (.228)
0.51 (.020) 0.33 (.013)
1.27 (.050) BSC
1.75 (.069) MAX 1.57 (.062) 1.37 (.054) 1.27 (.050) 0.40 (.016) 0.25 (.010) 0.19 (.008) D REF: JEDEC MS-012
0.25 (0.10) 0.10 (.004)
Plastic DIP (N); 300 mil wide
7.11 (.280) 6.10 (.240)
8.26 (.325) 7.62 (.300) 3.68 (.145) 2.92 (.115)
1.77 (.070) 1.14 (.045)
2.54 (.100) BSC
.356 (.014) .203 (.008)
0.39 (.015) MIN. .558 (.022) .356 (.014) Some 8 and 16 lead packages may have 1/2 lead at the end of the package. All specs are the same.
REF: JEDEC MS-001
D
Ordering Information
Part Number CS2841BEN8 CS2841BED14 CS2841BEDR14
Description 8L PDIP 14L SO Narrow 14L SO Narrow (tape & reel) 6
Cherry Semiconductor Corporation reserves the right to make changes to the specifications without notice. Please contact Cherry Semiconductor Corporation for the latest available information.
(c) 1999 Cherry Semiconductor Corporation
Rev. 6/23/99

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