Texas Instruments 0A Evaluation Module Featuring the TPS51117 Synchronous Buck Controller with D-CAP Mode TPS51117EVM TPS51117EVM Data Sheet
Product codes
TPS51117EVM
V
OUT
+
ǒ
1
)
R
1
R
2
Ǔ
0.75 V
I
OUT(LL)
+
1
2
L
ƒ
sw
ǒ
V
IN
*
V
OUT
Ǔ
V
OUT
V
IN
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SLVS631B – DECEMBER 2005 – REVISED SEPTEMBER 2009
DETAILED DESCRIPTION (continued)
At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or becomes ON state. This
MOSFET is turned off, or becomes OFF state, after the internal one shot timer expires. This one shot is
determined by V
MOSFET is turned off, or becomes OFF state, after the internal one shot timer expires. This one shot is
determined by V
IN
and V
OUT
to keep the frequency fairly constant over the input voltage range at steady state,
hence it is called adaptive on-time control or fixed frequency emulated on-time control (see PWM frequency and
Adaptive On-Time Control). The MOSFET is turned on again when both feedback information, monitored at V
Adaptive On-Time Control). The MOSFET is turned on again when both feedback information, monitored at V
FB
voltage, indicates insufficient output voltage AND inductor current information indicates below the overcurrent
limit. Repeating the operation in this manner, the controller regulates the output voltage. The synchronous
low-side or rectifying MOSFET is turned on each OFF state to keep the conduction loss to a minimum.
limit. Repeating the operation in this manner, the controller regulates the output voltage. The synchronous
low-side or rectifying MOSFET is turned on each OFF state to keep the conduction loss to a minimum.
The TPS51117 supports selectable PWM-only and auto-skip operation modes. If EN_PSV is grounded, the
switching regulator is disabled. If the EN_PSV pin is connected to 3.3 V or 5 V, the regulator is enabled with
auto-skip mode selected. The rectifying MOSFET is turned off when inductor current information detects zero
level. This enables a seamless transition to reduced frequency operation during a light load condition so that high
efficiency is maintained over a broad range of load currents. If the EN_PSV pin is floated, it is internally pulled up
to 1.95 V, and the regulator is enabled with PWM-only mode selected. The rectifying MOSFET is not turned off
when inductor current reaches zero. The converter runs forced continuous conduction mode for the entire load
range. System designers may want to use this mode to avoid a certain frequency during a light load condition but
with the cost of low efficiency. However, be aware the output has the capability to both source and sink current in
this mode. If the output terminal is connected to a voltage source higher than the regulator’s target, the converter
sinks current from the output and boosts the charge into the input capacitor. This may cause unexpected high
voltage at VIN and may damage the power FETs.
switching regulator is disabled. If the EN_PSV pin is connected to 3.3 V or 5 V, the regulator is enabled with
auto-skip mode selected. The rectifying MOSFET is turned off when inductor current information detects zero
level. This enables a seamless transition to reduced frequency operation during a light load condition so that high
efficiency is maintained over a broad range of load currents. If the EN_PSV pin is floated, it is internally pulled up
to 1.95 V, and the regulator is enabled with PWM-only mode selected. The rectifying MOSFET is not turned off
when inductor current reaches zero. The converter runs forced continuous conduction mode for the entire load
range. System designers may want to use this mode to avoid a certain frequency during a light load condition but
with the cost of low efficiency. However, be aware the output has the capability to both source and sink current in
this mode. If the output terminal is connected to a voltage source higher than the regulator’s target, the converter
sinks current from the output and boosts the charge into the input capacitor. This may cause unexpected high
voltage at VIN and may damage the power FETs.
DC output voltage can be set by the external resistor divider as follows (refer to
, and
).
(1)
LIGHT LOAD CONDITION WITH AUTO-SKIP FUNCTION
If auto-skip mode is selected, the TPS51117 automatically reduces the switching frequency during a light load
condition to maintain high efficiency. This reduction of frequency is achieved smoothly and without an increase of
V
condition to maintain high efficiency. This reduction of frequency is achieved smoothly and without an increase of
V
out
ripple or load regulation. Detailed operation is described as follows. As the output current decreases from a
heavy load condition, the inductor current is also reduced and eventually comes to the point that its valley
touches zero current, which is the boundary between continuous conduction and discontinuous conduction
modes. The rectifying MOSFET is turned off when this zero inductor current is detected. Since the output voltage
is still higher than the reference at this moment, both high-side and low-side MOSFETs are turned off and wait
for the next cycle. As the load current decreases further, the converter runs in discontinuous conduction mode,
taking longer time to discharge the output capacitor below the reference voltage. Note the ON time is kept the
same as during the heavy load condition. In reverse, when the output current increases from a light load to a
heavy load, the switching frequency increases to the preset value as the inductor current reaches to the
continuous conduction. The transition load point to light load operation, I
touches zero current, which is the boundary between continuous conduction and discontinuous conduction
modes. The rectifying MOSFET is turned off when this zero inductor current is detected. Since the output voltage
is still higher than the reference at this moment, both high-side and low-side MOSFETs are turned off and wait
for the next cycle. As the load current decreases further, the converter runs in discontinuous conduction mode,
taking longer time to discharge the output capacitor below the reference voltage. Note the ON time is kept the
same as during the heavy load condition. In reverse, when the output current increases from a light load to a
heavy load, the switching frequency increases to the preset value as the inductor current reaches to the
continuous conduction. The transition load point to light load operation, I
OUT(LL)
(i.e., the threshold between
continuous and discontinuous conduction mode), can be calculated as follows:
(2)
where f
sw
is the PWM switching frequency.
Switching frequency versus output current in the light load condition is a function of L, f
sw
, V
IN
and V
OUT
, but it
decreases almost proportional to the output current from the I
OUT(LL)
given above. For example, it is about 60 kHz
at I
OUT(LL)
/5 if the PWM switching frequency is 300 kHz.
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7
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