SG6905SZ

Manufacturer Part NumberSG6905SZ
DescriptionIC PWM CTLR PFC/FLYBACK 20-SOP
ManufacturerFairchild Semiconductor
SG6905SZ datasheet
 

Specifications of SG6905SZ

ModeAverage CurrentFrequency - Switching65kHz
Current - Startup10µAVoltage - Supply16 V ~ 20 V
Operating Temperature-40°C ~ 105°CMounting TypeSurface Mount
Package / Case20-SOIC (7.5mm Width)Lead Free Status / RoHS StatusLead free / RoHS Compliant
1
2
3
4
5
6
7
8
9
10
11
Page 11
12
Page 12
13
Page 13
14
Page 14
15
Page 15
16
Page 16
17
Page 17
18
Page 18
19
Page 19
20
Page 20
Page 18/20

Download datasheet (551Kb)Embed
PrevNext
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
.
PK
The voltage of V
determines the voltage of V
RMS
relationship between V
and VRMS is also shown in
PK
Figure 28.
The amplitude of the constant current I
by the internal current reference according to the
following equation:
(I
R
- )
0.2V
=
P
P
I
SENSE_peak
R
S
therefore the peak current of the I
(V
<1.05V)
RMS
Figure 28. V
Controlled Current Limiting
RMS
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.
Figure 29. Peak Current Control Loop
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
© 2007 Fairchild Semiconductor Corporation
SG6905 • Rev. 1.1.2
To
improve
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
. The
PK
voltage is higher than a designed threshold, FB
(4.5V), for longer than t
LOOP
OPWM is turned off. As OPWM is turned off, the supply
is determined
voltage V
P
DD
When V
is lower than the turn-off threshold, V
DD
(10V), SG6905 is totally shut down. Due to the startup
resistor, V
DD
voltage, V
(7)
DD-ON
the overloading condition still exists, the protection
takes place repeatedly. This prevents the power supply
is given by
SENSE
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
is equal to 24kΩ, the magnitude of the constant
I
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.
PD-PWM
18
stability
or
prevent
sub-harmonic
(56ms), the
OPEN-PWM
begins decreasing.
is charged up to the turn-on threshold
(16V), until SG6905 is enabled again. If
conditions.
OTP-OFF
(1.4V).
OTP-ON
Figure 30. OTP Function
www.fairchildsemi.com
OPEN-
DD-OFF
(1.2V),