Texas Instruments TPS23751 Evaluation Module TPS23751EVM-104 TPS23751EVM-104 데이터 시트
제품 코드
TPS23751EVM-104
SLVSB97C – JULY 2012 – REVISED JANUARY 2014
Current Slope Compensation and Current Limit
Current-mode control requires addition of a compensation ramp to the sensed inductive (transformer or inductor)
current for stability at duty cycles near and over 50%. The TPS23751 and TPS23752 have a maximum duty
cycle limit of 80%, permitting the design of wide input-range converters with lower voltage stress on the output
rectifiers. While the maximum duty cycle is 80%, converters may be designed that run at duty cycles well below
this for a narrower, 36 V to 57 V PI range. The TPS23751 and TPS23752 provide fixed internal slope
compensation which suffices for most applications.
current for stability at duty cycles near and over 50%. The TPS23751 and TPS23752 have a maximum duty
cycle limit of 80%, permitting the design of wide input-range converters with lower voltage stress on the output
rectifiers. While the maximum duty cycle is 80%, converters may be designed that run at duty cycles well below
this for a narrower, 36 V to 57 V PI range. The TPS23751 and TPS23752 provide fixed internal slope
compensation which suffices for most applications.
The TPS23751 and TPS23752 provide internal, frequency independent, slope compensation (V
PK
= 40 mV at
D
MAX
) starting from D
SLOPE_ST
to the PWM comparator input for current-mode control-loop stability. This voltage is
not applied to the current-limit comparator whose threshold is 0.25 V (V
CSMAX
). If the provided slope is not
sufficient, the effective slope may be increased by addition of R
S
per
. The additional slope voltage is
provided by (I
CS_RAMP
× R
S
). There is also a small dc offset caused by the I
CSDC
(approximately 2.0
μA) current.
The peak current limit does not have duty cycle dependency unless R
S
is used which is easier designing the
current limit to a fixed value. See the Current Slope Compensation section for more information.
The internal comparators monitoring CS are isolated from the CS pin by the blanking circuits while GATE is low,
and for a short time (blanking period) just after GATE switches high. A 500
and for a short time (blanking period) just after GATE switches high. A 500
Ω (max) equivalent pull-down resistor
on CS is applied while GATE is low.
RT
The RT pin programs the (free-running) oscillator frequency of the TPS23751 and TPS23752 in PWM mode. The
internal oscillator sets the maximum duty cycle at 80% and controls the slope-compensation ramp circuit. In VFO
mode, the RT pin is driven by V
internal oscillator sets the maximum duty cycle at 80% and controls the slope-compensation ramp circuit. In VFO
mode, the RT pin is driven by V
CTL
.
T2P, Startup and Power Management
T2P (type 2 PSE) is an active-low multifunction pin that indicates if
[(PSE = Type_2) + (V
APD
> 1.5V) + (V
CTL
< 4 V) × (PD current limit
≠ Inrush)].
The term with V
CTL
prevents an optocoupler connected to the secondary-side from loading V
C
before the
converter is started. The APD term allows the PD to operate from an adapter at high-power if a type 2 PSE is not
present, assuming the adapter has sufficient capacity. Applications must monitor the state of T2P to detect power
source transitions. Transitions could occur when a local power supply is added or dropped or when a PSE is
enabled on the far end. The PD may be required to adjust the load appropriately. The usage of T2P is
demonstrated in
present, assuming the adapter has sufficient capacity. Applications must monitor the state of T2P to detect power
source transitions. Transitions could occur when a local power supply is added or dropped or when a PSE is
enabled on the far end. The PD may be required to adjust the load appropriately. The usage of T2P is
demonstrated in
In order for a type 2 PD to operate at less than 13W the first 80ms after power application, the various delays
must be estimated and used by the application controller to meet the requirement. The bootup time of many
applications processors may be long enough to eliminate the need to do any timing.
must be estimated and used by the application controller to meet the requirement. The bootup time of many
applications processors may be long enough to eliminate the need to do any timing.
illustrates the T2P
delay after the converter starts.
Thermal Shutdown
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, and
forces the V
C
control into an under-voltage state.
Adapter ORing
Many PoE-capable devices are designed to operate from either a wall adapter or PoE power. A local power
solution adds cost and complexity, but allows a product to be used if PoE is not available in a particular
installation. While most applications only require that the PD operate when both sources are present, the
TPS23751 and TPS23752 supports forced operation from either of the power sources.
solution adds cost and complexity, but allows a product to be used if PoE is not available in a particular
installation. While most applications only require that the PD operate when both sources are present, the
TPS23751 and TPS23752 supports forced operation from either of the power sources.
illustrates three
options for diode ORing external power into a PD. Only one option would be used in any particular design.
Option 1 applies power to the TPS23751 and TPS23752 PoE input, option 2 applies power between the
TPS23751 and TPS23752 PoE section and the power circuit, and option 3 applies power to the output side of
the converter. Each of these options has advantages and disadvantages. Many of the basic ORing configurations
and much of the discussion contained in the application note Advanced Adapter ORing Solutions using the
TPS23753 (literature number
Option 1 applies power to the TPS23751 and TPS23752 PoE input, option 2 applies power between the
TPS23751 and TPS23752 PoE section and the power circuit, and option 3 applies power to the output side of
the converter. Each of these options has advantages and disadvantages. Many of the basic ORing configurations
and much of the discussion contained in the application note Advanced Adapter ORing Solutions using the
TPS23753 (literature number
), apply to the TPS23751 and TPS23752.
28
Copyright © 2012–2014, Texas Instruments Incorporated
Product Folder Links: