Datasheet

R
FBU
R
FBL
TLV431
R
OB
C
IZ
R
SS
C
SS
D
SS
FromRegulated
OutputVoltage
TPS23753
SLVS853C –JUNE 2008–REVISED JANUARY 2010 www.ti.com
Care in design of the transformer and V
C
bias circuit is required to obtain hiccup overload protection.
Leading-edge voltage overshoot on the bias winding may cause V
C
to peak-charge, preventing the expected
tracking with output voltage. R
VC
(Figure 1) is often required slow the peak charging. Good transformer
bias-to-output-winding coupling results in reduced overshoot and better voltage tracking.
The startup current source transitions to a resistance as (V
DD1
- V
C
) falls below 7 V, but will start the converter
from 12 V adapters within t
ST
(V
DD1
≥ 10.2, V~85 ms). The bootstrap source provides reliable startup from widely
varying input voltages, and eliminates the continual power loss of external resistors. The startup current source
will not charge above the maximum recommended V
VC
if the converter is disabled and there is sufficient V
DD1
to
charge higher.
The peak current limit does not have duty cycle dependency unless R
S
is used as shown in Figure 22 to increase
slope compensation. This makes it easier to design the current limit to a fixed value.
The TPS23753 blanker timing is precise enough that the traditional R-C filters on CS can be eliminated. This
avoids current-sense waveform distortion, which tends to get worse at light output loads. While the internally set
blanking period is relatively precise, almost all converters will require their own blanking period. The TPS23753
provides the BLNK pin to allow this programming. There may be some situations or designers that prefer an R-C
approach. The TPS23753 provides a pull-down on CS during the GATE off time to improve sensing when an
R-C filter must be used. The CS input signal should be protected from nearby noisy signals like GATE drive and
the MOSFET drain.
Converters require a softstart on the voltage error amplifier to prevent output overshoot on startup. Figure 21
shows a common implementation of a secondary-side softstart that works with the typical TL431 error amplifier
shown in Figure 1. This secondary-side error amplifier will not become active until there is sufficient voltage on
the secondary. The TPS23753 provides a primary-side softstart which persists long enough (~800 μs) for
secondary side voltage-loop softstart to take over. The primary-side current-loop softstart controls the switching
MOSFET peak current by applying a slowly rising ramp voltage to a second PWM control input. Figure 19 shows
an exaggerated handoff between the primary and secondary-side softstart that is most easily seen in the I
PI
waveform. The output voltage rises in a smooth monotonic fashion with no overshoot. This handoff can be
optimized by decreasing the secondary-side softstart period.
Figure 21. Example of Softstart Circuit Added to Error Amplifier
The dc/dc controller has an OTSD that can be triggered by heat sources including the V
B
regulator, GATE driver,
bootstrap current source, and bias currents. The controller OTSD turns off V
B
, the GATE driver, resets the
softstart generator, and forces the V
C
control into an under-voltage state.
Special Switching MOSFET Considerations
Special care must be used in selecting the converter switching MOSFET. The TPS23753 converter section has
minimum V
C
operating voltage of ~5.5 V, which is reflected in the applied gate voltage. This will occur during an
output overload, or towards the end of a (failed) bootstrap startup. The MOSFET must be able to carry the
anticipated peak fault current at this gate voltage.
18 Submit Documentation Feedback Copyright © 2008–2010, Texas Instruments Incorporated
Product Folder Link(s): TPS23753