Datasheet
( ) ( )
( )
-
= ´ + ´ +
1
OSC CT RTC RTD
f 0.74 C 27pF R R
( )
= ´ ´ + ´
MAX TC T OSC
D 0.74 R C 27pF f
+
–
+
–
S Q
R
Q2
Q1
3
4
R
RTD
C
CT
R
RTC
V
REF
3.33 V
1.67 V
CLK
Oscillator
Oscillator
Latch
UDG-10005
RTC
RTD
TPS92001, TPS92002
SLUSA24A –FEBRUARY 2010–REVISED NOVEMBER 2010
www.ti.com
APPLICATION INFORMATION
Introduction
The typical application diagrams in Figure 3 and Figure 4 show isolated and non-isolated flyback converters
utilizing the TPS92001. Note that the capacitors C
REF
and C
VDD
are local decoupling capacitors for the reference
and device input voltage, respectively. Both capacitors should be low ESR and ESL ceramic, placed as close as
possible to the device pins, and returned directly to the ground pin of the device for best stability. The REF pin
provides the internal bias to many of the device functions and C
REF
should be at least 0.47-µF to prevent the
REF voltage from drooping.
Current Sense (CS) Pin
In the TPS92001/2, the current regulation is obtained through the summation of the primary current sense and
any slope compensation at the CS pin compared to a 1-V threshold, as shown in the FUNCTIONAL BLOCK
DIAGRAM. Crossing this 1-V threshold resets the PWM latch and modulates the output driver on-time. In the
absence of a CS signal, the output obeys the programmed maximum on-time of the oscillator. When adding
slope compensation, it is important to use a small capacitor to AC couple the oscillator waveform before
summing this signal into the CS pin. By forcing the CS node to exceed the 1-V threshold the TPS92001/2 is
forced to zero percent duty cycle.
Oscillator
Equation 3 calculates the oscillator frequency setting.
(3)
(4)
Referring to Figure 1 and the waveforms in Figure 2, when Q1 is on, C
CT
charges via the on-resistance of the Q1
MOSFET and the RTC pin. During this charging process, the voltage of C
CT
is sensed through the RTD pin. The
S input of the oscillator latch, S
OSC
, is level sensitive, so crossing the upper threshold (set at 2/3 VREF or 3.33 V
for a typical 5.0 V reference) sets the Q output (CLK signal) of the oscillator latch high. A high CLK signal results
in turning off Q1 and turning on Q2. The timing capacitor then discharges through RTD and the R
DS(on)
of Q2.
C
CT
discharges from 3.33 V to the lower threshold (set at 1/3 REF or 1.67 V for a typical 5.0-V reference) sensed
through RTC. The R input to the oscillator latch, R
OSC
, is also level sensitive and resets the CLK signal low when
C
CT
crosses the 1.67-V threshold, turning off Q2 and turning on Q1, initiating another charging cycle.
Figure 1. Oscillator Function
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Product Folder Link(s): TPS92001 TPS92002