L6668 STMICROELECTRONICS | Alldatasheet
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Technical content
This is preliminary information on a new product now in development or undergoing evaluation. Details are subject to change wit hout notice.
1 General Features
1.1 APPLICATIONS
Figure 2. Block Diagram
16 Vref
Figure 1. Package Table 1. Order Codes
2 Description
L6668 is a current-mode primary controller IC, designed to build single-ended converters. ables to turn off the pre-regulator when the load level falls below a threshold. in case the secondary diode fails short. Table 2. Absolute Maximum Ratings Table 3. Thermal Data Figure 3. Pin Connection (Top view)
Table 4. Pin Description 0.5V below the start-up threshold. comply with safety regulations (creepage distance) on the PCB. and kept separate from any pulsed current return. high values when the IC is supplied with a voltage close to or exceeding 20V . clean bias voltage for the signal part of the IC. 6 N.C. Connect the pin to GND. the UVLO threshold. Connect the pin to GND if the function is not used. stability of the generator and to prevent noise from affecting the reference. the function, if the pin is pulled below 0.8V the IC is shut down. pin SKIPADJ shuts down the IC and reduces its current consumption. quickly discharged as the chip goes into UVLO. device down and brings its consumption almost to a “before start-up” level.
oscillator allows programming frequency foldback at light load. Table 5. Electrical Characteristcs Table 4. Pin Description (continued)
Table 5. Electrical Characteristcs (continued)
Figure 4. Typical System Block Diagram
3 Typical Electrical Performance
Figure 5. High-voltage generator ON-state sink Figure 6. High-voltage generator output (Vcc Figure 7. High-voltage pin leakage vs. Tj Figure 8. High-voltage generator start voltage Figure 9. High-voltage generator Vcc restart Figure 10. IC consumption vs. Tj
4 Application Information
mode-controlled flyback converters. pulses and is turned off by the control loop. duced as well by slowing down the charge of the timing capacitor proportionally to the load itself. low, no issue of audible noise arises. Figure 34. Multi-mode operation of the L6668
4.1 High-voltage start-up generator
pensated current generator connected to its source. Figure 35. High-voltage start-up generator: internal schematic stand 700V, controlled by a low-voltage chip, where all of the control functions reside.
below the UVLO threshold (8.7V typ.). too and the HV generator is disabled. This prevents converter's re-start attempts and ensures monotonic output voltage decay at power-down. work safely with extremely low power throughput.
4.2 Frequency Foldback Block and operation at medium/light load
frequency like a standard current mode PWM controller. load conditions - the lower the load, the lower the frequency and vice versa. Figure 38. Frequency foldback function: oscillator frequency is a function of COMP voltage vates the circuit shown in figure 38.
Figure 39. Standby function: frequency shift vs. timing resistors (normalized quantities) a maximum frequency shift allowed. Not observing these limits will result in erratic behavior. no major drawback to this technique. In case this function is not desired, the STBY pin shall be left open.
4.3 Operation at no load or very light load
duced at a very low value to minimize Vcc capacitor discharge. The soft-start capacitor is not discharged. current, which is user-defined by the bias voltage at pin 9, is very low.
Figure 40. Load-dependent operating modes: timing diagram sure that the voltage on the pin does not linger on the threshold to prevent uncertain behavior.
4.4 PWM control Block
lector (the emitter is grounded to GND) to modulate the duty cycle.
4.5 Current Comparator, PWM Latch and Hiccup-mode OCP
switching of the MOSFET, which might result from the noise generated ("double-pulse suppression").
Figure 41. Hiccup-mode OCP: timing diagram the secondary rectifier or a shorted secondary winding or a saturated flyback transformer.
4.6 Power Management
factor-corrected systems where a PFC pre-regulator precedes the DC-DC converter. be turned off, thus saving the no-load consumption of this stage (0.5 to 1W). becomes open when the voltage VCOMP falls below 2.2V. figure 42. To prevent intermittent operation of the PFC stage, 0.5V hysteresis is provided: the PFC_STOP pin is re-asserted low (which will re-enable the PFC pre-regulator) when VCOMP exceeds 2.7 V. will start up only after the DC-DC converter governed by the L6668 is activated.
4.7 Disable function
below the UVLO threshold to reset the latch and restart the IC. proximately by three and keeps peak silicon temperature close to the average value.
4.8 Slope compensation
tion of the oscillator sawtooth, buffered (0.8 mA min. capability) and level shifted down by one Vbe. continuous conduction mode with a duty cycle close to or exceeding 50%. Figure 45. Slope compensation waveforms not required the pin shall be left floating.
Figure 46. Typical Application: 80W, WRM flyback; Electrical Schematic Table 6. Light load measurements on the circuit of figure 46
5 Package information
is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an STMicroelectronics trademark. ECOPACK specifications are available at: www.st.com.
Figure 47. SO16 (Narrow) Mechanical Data & Package Dimensions
6 Revision History
Table 7. Revision History
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners © 2005 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America L6668