L6668 STMicroelectronics, L6668 Datasheet - Page 13

• Manufacturer Part Number: L6668
• Description: IC CTRLR PWM PROG CM HV 16SOIC
• Manufacturer: STMicroelectronics

Specifications of L6668

• Output Isolation: Isolated
• Frequency Range: 93 ~ 107kHz
• Voltage - Input: 9.4 ~ 22 V
• Power (watts): 750mW
• Operating Temperature: -25°C ~ 150°C
• Package / Case: 16-SOIC (0.154", 3.90mm Width)
• Number Of Outputs: 1
• Duty Cycle (max): 75 %
• Output Current: 800 mA
• Mounting Style: SMD/SMT
• Switching Frequency: 105 KHz
• Maximum Operating Temperature: + 150 C
• Fall Time: 30 ns
• Minimum Operating Temperature: - 25 C
• Rise Time: 55 ns
• Synchronous Pin: No
• Topology: Flyback
• Load-dependent Current-mode Control: FIXED-FREQUENCY (HEAVY LOAD), FREQUENCY FOLDBACK (LIGHT LOAD), BURST-MODE (NO-LOAD)
• For Use With: 497-6389 - BOARD EVAL USING L6668 & STSR30497-5499 - EVAL BOARD FOR L6668
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

With reference to the timing diagram of figure 36, when power is first applied to the converter the voltage
on the bulk capacitor (Vin) builds up and, as it reaches about 80V, the HV generator is enabled to oper-
ate (HV_EN is pulled high) so that it draws about 1 mA. This current, diminished by the IC consumption,
charges the bypass capacitor connected between pin Vcc (5) and ground and makes its voltage rise al-
most linearly.
Figure 36. Timing diagram: normal power-up and power-down sequences
As the Vcc voltage reaches the start-up threshold (13.5V typ.) the low-voltage chip starts operating and
the HV generator is cut off by the Vcc_OK signal asserted high. The IC is powered by the energy stored
in the Vcc capacitor until the self-supply circuit (typically an auxiliary winding of the transformer and a
steering diode) develops a voltage high enough to sustain the operation.
The residual consumption of this circuit is just the one on the 15M Ω resistor ( ≈ 10 mW at 400 Vdc), typically
50-70 times lower, under the same conditions, as compared to a standard start-up circuit made with an
external dropping resistor.
Figure 37. Timing diagram showing short-circuit behavior
Vcc restart
Vcc restart
V HVstart
V HVstart
Vcc OFF
Vcc OFF
Vcc_OK
Vcc_OK
Vcc ON
Vcc ON
HV_EN
HV_EN
OUT
OUT
1 mA
1 mA
Vcc
Vcc
Vin
Vin
I HV
I HV
Vcc_OK
Vcc_OK
Vcc
Vcc
Vcc
Vcc
Vcc
Vcc
OUT
OUT
1 mA
1 mA
Vcc
Vcc
I
I
OFF
OFF
HV
HV
rest
rest
ON
ON
Power-up
Power-up
Short circuit occurs here
Short circuit occurs here
operation
operation
Normal
Normal
Power-down
Power-down
regulation is lost here
regulation is lost here
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
L6668
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