Texas Instruments | Understanding 100% mode in low-power DC/DC converters | Application notes | Texas Instruments Understanding 100% mode in low-power DC/DC converters Application notes

Texas Instruments Understanding 100% mode in low-power DC/DC converters Application notes
Analog Design Journal
Power
Understanding 100% mode in low-power
DC/DC converters
By Chris Glaser
Member Group Technical Staff, Senior Applications Engineer
Introduction
Figure 1. Power-stage for a typical low-power,
synchronous step-down converter
With their low quiescent current (IQ) and small total-­
solution size, low-power step-down DC/DC converters
such as the TPS62xxx series are typically optimized for
battery-powered portable applications.[1] The majority of
these converters also support a 100% duty-cycle mode
(100% mode), where the high-side MOSFET turns on for
100% of the time to create a direct path from the input
voltage through the inductor to the output voltage. This
maintains sufficient output voltage even as the battery
discharges and its voltage drops to levels just above the
output voltage. The 100% mode minimizes this dropout
voltage and allows the highest possible output voltage
from a step-down converter. In 100% mode, the input
voltage is reduced only by the high-side MOSFET’s and
the inductor’s ohmic losses and is not further reduced by
the duty cycle of the high-side MOSFET. This article
explains the 100% mode of low-power DC/DC converters
and compares it with other 100%-mode implementations.
CP
Charge Pump
for
Gate Driver
Hiccup
Current Limit
# 32 Counter
High-Side
Current
Sense
M1
MOSFET Driver
Anti-Shoot-Through
Converter Control
Logic
Step-down converter operation
VGS_M1
SW
SW
M2
Figure 1 shows the basic block diagram for the power
stage of the TPS62090 low-power synchronous step-down
converter. The switch pins (SW) connect to the output
filter (inductor and capacitor), which generates the regulated output voltage.
When the high-side MOSFET (M1) is on, the voltage on
the SW pins becomes the same voltage that is on the
power-input voltage (PVIN) pins, slightly reduced by M1’s
ohmic losses. For an N-channel MOSFET, as shown in
Figure 1, the gate driver applies a voltage higher than the
SW pins, M1’s source terminal, to generate the required
positive gate-source voltage (VGS_M1). For a comparable
circuit with a P-channel MOSFET, the gate driver applies a
voltage lower than the PVIN pins, the high-side MOSFET’s
source terminal, to generate the required negative VGS.
Generally, higher-current DC/DC converters use an
N-channel MOSFET for the high-side MOSFET because of
the higher electron-charge-carrier conductivity and mobil­
ity compared to P-channel MOSFET’s hole-charge-carrier
conductivity and mobility. Thus, lower drain-source resistance (RDS(on)) is achieved with N-channel MOSFETs
compared to P-channel MOSFETs.[2, 3] Regardless of
MOSFET type, the DC/DC converter uses appropriate
gate-driving techniques to achieve the operation described
in the specific device data sheet, which includes 100%
mode for many devices.
Texas Instruments
PVIN PVIN
CN
PGND
PGND
Bootstrap capacitor
Driving the gate voltage above the source voltage for an
N-channel high-side MOSFET requires additional circuitry,
because the source voltage (SW pins) is at the level of the
input voltage (PVIN pins) and there is no higher voltage
available in the step-down converter. A bootstrap capacitor typically creates the required higher voltage to drive
the high-side N-channel MOSFET. For the TPS62090, the
bootstrap capacitor is placed between the CP and CN pins
shown in Figure 1. For most other low-power devices,
integrating the bootstrap capacitor entirely inside the
DC/DC converter (on the same die as the MOSFETs) gives
the least parasitics to improve normal operation and offers
the best 100%-mode performance.
Lower-current devices sometimes use a P-channel highside MOSFET, which does not require a bootstrap capacitor since no higher voltage is required to turn it on.
Appropriate bootstrap-capacitor design techniques ensure
that a device with an N-channel high-side MOSFET has
the same 100%-mode performance as a DC/DC converter
with a P-channel high-side MOSFET.
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VOUT
R1
R2
Voltage (2 V/div)
Voltage (2 V/div)
Many devices use an N-channel high-side
Figure 2. Typical DC/DC converter with an
MOSFET and an external capacitor between the
external bootstrap capacitor, CBoot
BOOT and phase (PH) pin to reduce die size and
cost. As the converter’s current increases, the highside MOSFET size and gate charge also increase.
PVIN
VIN
This requires larger bootstrap capacitor values,
VIN
CBoot
TPS54623
which are not practical to integrate inside the
CIN
BOOT
converter. Figure 2 shows the placement of the
external bootstrap capacitor, CBoot, for the
LO
EN
PH
TPS54623. The PH pin is equivalent to the SW pin.
Fundamentally, a bootstrap capacitor first
CO
PWRGD
charges to some voltage, typically either the input
voltage or a lower voltage created internally by the
VSENSE
SS/TR
converter, while one of its terminals is at GND. This
RT/CLK
same terminal is then connected to the PH pin,
COMP
GND
CSS
which boosts the other capacitor terminal above
C2
Exposed
R3
the PH pin voltage by the voltage to which the
RRT
Thermal Pad
C1
capacitor was originally charged. The capacitor
holds this voltage for some time, as it supplies
charge to the gate of the high-side MOSFET.
However, leakage currents reduce this stored
charge, and the bootstrap capacitor must recharge
to keep the high-side MOSFET on. Proper sizing of the
Figure 3. Comparison of two configurations
bootstrap capacitor value maintains sufficient charge for
for bootstrap capacitors
the duration of the switching period, at which point the
bootstrap capacitor recharges.
With the bootstrap circuit shown in Figure 2, the bootBOOT
strap capacitor is always connected to the PH pin. It
recharges when PH is at GND potential, which only occurs
when the low-side MOSFET is on, and forces the charging
VIN
PH
(Yellow)
of the bootstrap capacitor to be coincident with the
switching of the power MOSFETs. This is not the case with
internal bootstrap capacitor devices, which do not permaVOUT
nently connect one terminal of the bootstrap capacitor to
the SW pin. Therefore, the bootstrap capacitor charging is
independent from the switching action and can still
recharge in 100% mode.
Figure 3 compares the two bootstrap-capacitor configuTime (1 µs/div)
rations. The TPS54623’s bootstrap capacitor is charged
(a) TPS54623 bootstrap-capacitor charging
when the BOOT trace goes below the VIN trace, which occurs
is coincident with switching
when the PH trace is low. In 100% mode, the TPS62090
recharges its bootstrap capacitor without requiring any
switching on the SW pin. The TPS62090’s CP trace is
CP
equivalent to the TPS54623’s BOOT trace. Both go above
the input voltage to drive the high-side MOSFET’s gate.
100%-mode operation
A very important difference between the implementations
in Figure 1 and Figure 2 is the connection of the bootstrap
capacitor. The circuit in Figure 1 (and all other devices
with internal bootstrap capacitors) controls both terminals
of the bootstrap capacitor. Alternatively, the circuit in
Figure 2 controls just one pin, while sharing the PH pin
with the inductor and internal power MOSFETs for the
capacitor’s second terminal. Unlike the TPS54xxx device,
the TPS62xxx device has complete control over where the
bootstrap capacitor connects and when it recharges.
Texas Instruments
SW
VIN
(Yellow)
CN
Time (50 µs/div)
(b) TPS62090 bootstrap-capacitor charging still occurs in
100% mode without switching
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Power
Having control over when the bootstrap capacitor
recharges is critical in many battery-powered applications
when the battery voltage decreases to just above the
desired output voltage. In such cases, the converter needs
to maintain a high-enough output voltage to properly
power the load and thus keep the system operating.
Increasing the duty cycle to 100% and keeping the highside MOSFET on all the time achieves the highest output
voltage. Any off-time required to recharge a bootstrap
capacitor reduces the 100% duty cycle to some lower
value, which reduces the average output voltage and
creates additional output-voltage ripple.
Figure 4 compares the TPS54623 and TPS62135 when
operating from a deeply discharged two-cell lithium
battery at 5.0 V and creating a 5-V output voltage supplying 2 A of current. The DC output voltage for the
TPS54623 is slightly higher than the TPS62135 in this
dropout condition due to its much lower MOSFET RDS(on).
However, the TPS62135 creates a cleaner output voltage
without ripple because it does not need to switch to maintain its 100% mode.
Figure 5 shows the same two devices’ line regulation.
With no load, as shown in Figure 5a, the TPS54623
provides a lower output voltage because it is not switching
often enough to keep the bootstrap capacitor charged. In
Figure 5b with a 2-A load, the TPS54623 does switch often
enough to maintain the bootstrap capacitor’s charge and
outputs a higher output voltage due to its lower MOSFET
RDS(on). Figure 5 also shows the TPS563200 as a device
with a 65% maximum recommended duty cycle.
Also shown in Figure 5, the output voltage of the
TPS563200 begins to decrease at much higher input voltages due to its duty-cycle limit. Limiting the duty cycle to
levels far below 100% optimizes these devices for
Figure 4. Comparison of dropout conditions for two converters
VIN
(Yellow)
VOUT
(Red)
Voltage (1 V/div)
Voltage (1 V/div)
VIN
(Yellow)
SW
(Blue)
SW
(Blue)
VOUT
(Red)
Time (200 µs/div)
Time (2 µs/div)
(b) TPS62135 operating in 100% mode without switching
(a) TPS54623 operating in near-100% mode with switching
Figure 5. Line-regulation comparison of the TPS54623, TPS62135 and TPS563200
5.5
5.5
TPS54623
5
5
Output Voltage (V)
Output Voltage (V)
TPS54623
4.5
TPS62135
TPS563200
4
4.5
TPS62135
4
TPS563200
3.5
3.5
3
3
3
3.5
4
4.5
5
5.5
6
Input Voltage (V)
6.5
7
7.5
3
8
4
4.5
5
5.5
6
Input Voltage (V)
6.5
7
7.5
8
(b) With 2-A load
(a) With no load
Texas Instruments
3.5
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Analog Design Journal
Power
Figure 6. TPS62135 minimum off-time with reduced
switching frequency, before enterning 100% mode
VIN
(Yellow)
Voltage (1 V/div)
cost-effective applications. In these systems, the input
voltage is usually fixed at 12 V and therefore does not
require high duty cycles to generate the required lower
voltages. Finally, the TPS62135 operates down to a 3-V
input voltage, whereas the TPS54623 and TPS563200 are
rated to 4.5 V. This is important in backup power applications, where the converter provides power to the system
from a super capacitor. As the super capacitor provides
power, its voltage decreases. The lower input-voltage
capability provides an output voltage for a longer amount
of time, which extracts more energy from the super
capacitor.
VOUT
(Red)
SW
(Blue)
Operation at Near-100% mode
Many devices with external bootstrap capacitors support
100%-mode operation as long as the bootstrap capacitor
remains sufficiently charged, as measured by an undervoltage-lockout (UVLO) circuit on the bootstrap capacitor itself. This UVLO circuit is different than the UVLO
circuit on the input voltage and ensures that the bootstrap
capacitor is sufficiently charged to properly turn on the
high-side MOSFET. If the capacitor is not sufficiently
charged, it recharges by turning off the high-side
MOSFET. Thus, these devices do not support 100% mode
for lengthy times but do support a near-100% mode, as
shown in Figure 4. Reference 4 describes various methods
to obtain an improved 100% mode under some conditions
with TPS54xxx devices.
As the input voltage drops toward the output voltage,
most devices operate with a minimum off-time. This
minimum off-time is simply the shortest on-time of the
low-side MOSFET that the converter is able to generate.
Once this off-time is reached, the converter decreases its
switching frequency to maintain both the output voltage
and the minimum off-time. As the input voltage continues
dropping, the TPS62135 eventually transitions to 100%
mode with an off-time of 0 ns. The minimum off-time does
not prohibit operation at any specific duty cycle or operating
point, and simply refers to the point at which the switching
frequency begins to drop from its nominal value.[5] Figure 6
shows the 80-ns minimum off-time of the TPS62135.
Table 1 summarizes the typical 100%-mode performance
of various step-down converter devices. Consult the
device data sheet for details regarding a specific device.
Time (1 µs/div)
Conclusion
The true 100%-mode operation of most TPS62xxx devices
makes these DC/DC converters a good fit for batterypowered applications where the battery voltage drops to
just above the required output voltage. Their small size
and low IQ add to their suitability. Using an internal bootstrap capacitor, or using two pins for an external bootstrap
capacitor, enables the charging of the bootstrap capacitor
to be independent from the switching action. This is
different from most TPS54xxx devices, which only use a
single pin to connect to an external bootstrap capacitor.
While the TPS62xxx devices generally have very good
dropout performance, other devices can have maximum
duty-cycle limits that prohibit their use in high-duty-cycle
applications. It is important to read each device’s data
sheet to understand 100%-mode behavior if it is critical
for a given application.
References
1. Chris Glaser, “Iq: What it is, what it isn’t, and how to use
it,” Texas Instruments Analog Applications Journal
(SLYT412), 2Q 2011.
2. See the Wikipedia entry for MOSFET.
3. See the Wikipedia entry for electron mobility.
4. Jerry Chen, Steve Schnier, Anthony Fagnani and Dave
Daniels, “Methods to Improve Low Dropout Operation
with the TPS54240 and TPS54260,” Texas Instruments
application report (SLVA547A), October 2013.
5. Chris Glaser, “Understanding frequency variation in the
DCS-Control™ topology,” Texas Instruments Analog
Applications Journal (SLYT646), 4Q 2015.
Table 1. 100%-mode performance of various step-down converters
100%-Mode
Operation
Device
Applications
Bootstrap
Capacitor
True 100%
mode
TPS62xxx
Battery-powered
with small size
and low IQ
Internal or
e­ xternal with
2-pin control
Near-100%
mode
TPS54xxx
Higher currents
with lowest
RDS(on)
External, with
1-pin control
Recommended
<65%
TPS563xxx
Cost-effective
External, with
1-pin control
Texas Instruments
Related Web sites
Product information:
TPS62090
TPS62135
TPS54623
TPS563200
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