SG6905SZ Fairchild Semiconductor, SG6905SZ Datasheet - Page 18

• Manufacturer Part Number: SG6905SZ
• Description: IC PWM CTLR PFC/FLYBACK 20-SOP
• Manufacturer: Fairchild Semiconductor

Specifications of SG6905SZ

• Mode: Average Current
• Frequency - Switching: 65kHz
• Current - Startup: 10µA
• Voltage - Supply: 16 V ~ 20 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

© 2007 Fairchild Semiconductor Corporation
SG6905 • Rev. 1.1.2
Cycle-by-Cycle Current Limiting
SG6905 provides cycle-by-cycle current limiting for
both PFC and PWM stages. Figure 28 shows the peak
current limit for the PFC stage. The PFC gate drive is
terminated once the voltage on ISENSE pin goes below
V
The voltage of V
relationship between V
Figure 28.
The amplitude of the constant current I
by the internal current reference according to the
following equation:
therefore the peak current of the I
(V
Flyback PWM and Slope Compensation
As shown in Figure 29, peak-current-mode control is
utilized for flyback PWM. The SG6905 inserts a
synchronized 0.5V ramp at the beginning of each
switching cycle. This built-in slope compensation
ensures stable operation for continuous-current-mode
operation.
When the IPWM voltage across the sense resistor
reaches the threshold voltage (0.9V), the OPWM is
turned off after a small propagation delay t
I
SENSE_peak
PK
RMS
.
Figure 28. V
<1.05V)
Figure 29. Peak Current Control Loop
=
RMS
(I
RMS
P
•
determines the voltage of V
R
Controlled Current Limiting
R
P
PK
- )
S
and VRMS is also shown in
0.2V
SENSE
P
is determined
PD-PWM
is given by
PK
.
. The
(7)
To
oscillation, a synchronized positive-going ramp is
inserted at every switching cycle.
Limited Power Control
When the output power supply is shorted or over
loaded, the FBPWM voltage increases. If the FBPWM
voltage is higher than a designed threshold, FB
LOOP
OPWM is turned off. As OPWM is turned off, the supply
voltage V
When V
(10V), SG6905 is totally shut down. Due to the startup
resistor, V
voltage, V
the overloading condition still exists, the protection
takes place repeatedly. This prevents the power supply
from being overheated under overloading
Over-Temperature Protection (OTP)
SG6905 provides an OTP pin for over-temperature
protection. A constant current is output from this pin. If
R
current is 100µA. An external NTC thermistor must be
connected from this pin to ground as shown in Figure
30. When the OTP voltage drops below V
SG6905 is disabled, and does not recover until OTP
voltage exceeds V
Soft-Start
During startup of the PWM stage, the SS pin charges
an external capacitor with a constant current source.
The voltage on FBPWM is clamped by SS voltage
during startup. In the event of a protected condition
occurring and/or PWM being disabled, the SS pin is
quickly discharged.
Gate Drivers
The SG6905 output stage is a fast totem-pole gate
driver. The output driver is clamped by an internal 18V
Zener diode in order to protect the external power
MOSFET.
18
I
is equal to 24kΩ, the magnitude of the constant
improve
(4.5V), for longer than t
DD
DD
DD-ON
DD
is lower than the turn-off threshold, V
begins decreasing.
is charged up to the turn-on threshold
Figure 30. OTP Function
(16V), until SG6905 is enabled again. If
stability
OTP-ON
(1.4V).
or
prevent
OPEN-PWM
conditions.
sub-harmonic
OTP-OFF
(56ms), the
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(1.2V),
DD-OFF
OPEN-