Texas Instruments | TPS54618-Q1 High-Frequency (2.35 MHz) Operation | Application notes | Texas Instruments TPS54618-Q1 High-Frequency (2.35 MHz) Operation Application notes

Texas Instruments TPS54618-Q1 High-Frequency (2.35 MHz) Operation Application notes
Application Report
SLVA877 – March 2017
TPS54618-Q1 High-Frequency (2.35 MHz) Operation
John Tucker
ABSTRACT
This document describes the TPS54618-Q1 High-Frequency (2.35 MHz) operation and (2.5 MHz)
simulation test results. This application report is applicable to TPS57114-Q1 and TPS54388-Q1 devices
also as these two devices belong to the same family as the TPS54618-Q1 device.
1
Introduction
The TPS54618-Q1 device is a full-featured 6-Vin, 6-A, synchronous step-down current-mode DC-DC
converter with two integrated metal-oxide-semiconductor field-effect transistors (MOSFETs). The
TPS54618-Q1 device can operate in a wide range of switching frequencies from 200-KHz to 2-MHz. The
switching frequency can be set using a pulldown resistor or an externally applied signal at the RT pin as
shown in Figure 1. Refer to Interfacing TPS57xxx-Q1,TPS65320-Q1 Family, and TPS65321-Q1 Devices
With Low Impendence External Clock Drivers (SLVA755), for more information regarding interfacing an
external clock to TPS54618-Q1 family of devices.
TPS54618-Q1
RT/CLK
PLL
Clock
Source
Rt
Figure 1. Synchronizing to a High-Impedance External Clock Source
The regulated output voltage at no load is given by Equation 1:
Vout = Vin × tON × fSW
where
•
•
•
Vin = input voltage
Vout = output voltage
tON = ON time
•
fSW = switching frequency
and
(1)
For given input and output voltages, as switching frequency is increased, tON will reduce. If tON falls below
the minimum value allowed by the internal circuit design, the output voltage may not track the internal 0.8V reference and would therefore not be regulated. To maintain the regulation at higher switching
frequencies, tON must be kept above a minimum value. If the switching frequency is less than or equal to
the data sheet maximum value of 2 MHz, tON will always be above designed minimum value and the
output would be regulated.
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1
2.35-MHz Operation
2
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2.35-MHz Operation
Bench test and chip-level simulations were done to check whether the TPS54618-Q1 can remain in
regulation while switching at a frequency higher than 2.0 MHz.
The test conditions for the bench test and chip-level simulation were as follows:
• Supply input voltage: 5.33 V
• Regulated output voltage: 1.35 V
• Operating switching frequency in bench setup: 2.35 MHz synchronized to an external clock as shown
in Figure 1.
Under these test conditions, the on time at a 100-mA load for the TPS54618-Q1 to maintain regulation
was verified by bench measurements to be 109.6 ns, compared to the value estimated by Equation 2:
(1.35 V / 5.33 V) × (1 / 2.35 MHz) = 107.8 ns
(2)
Minimum controllable on time at higher frequency and input voltage is verified to be less than 90 ns,
confirmed by the simulations in Section 3.
3
2.5-MHz Operation Simulation Results
A full chip breaking-point simulation is performed at 2.5-MHz clock frequency using the input and output
voltages specified in the following list to determine the minimum controllable tON allowed by internal circuit
design. The 2.5-MHz clock frequency is chosen to have some margin for oscillator tolerances and to be
sure that it works for 2.35-MHz clock frequency.
• Input voltage swept from 5 V to 6 V
• Output voltage programmed to 1.35 V
• Load current set to 10 mA using a 135-Ω resistor
• LOUT = 0.33 µH, COUT = 33 µF
• RCOMP = 3.83 kΩ, CZERO = 1.5 nF, CPOLE = 0 pF
Simulation results:
• Transient turn on behavior appears normal
Table 1 lists the simulation results across process corners and across temperature with 10-mA load
current.
Table 1. Breaking Point Simulation Data With 10-mA Load at 2.5 MHz
Process Corner / Simulated VREF (mV)
Temperature
VIN (V)
Actual Vout (V)
FSW (MHz)
Calculated tON (ns)
Simulated tON (ns)
Nominal / 27°C
819
6
1.352
2.5
90.1
94.9
Weak / –40°C
814
6
1.343
2.5
89.5
94.5
Weak / 150°C
835
6
1.382
2.5
92.1
96.2
Strong / –40°C
809
6
1.335
2.5
89.0
91.8
Strong / 150°C
823
6
1.362
2.5
90.8
96.3
Based on the simulation results, the minimum controllable on-time is found to be lower than 100 ns at a
10-mA load.
2
TPS54618-Q1 High-Frequency (2.35 MHz) Operation
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Conclusion
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4
Conclusion
The switching ON time for an input voltage of 5.33 V and a regulated output voltage of 1.35 V and a
switching frequency of 2.35 MHz is calculated to be 107.8 ns during no load conditions. This is higher than
the minimum controllable ON time of 105 ns allowed by internal circuit design based on simulations. The
TPS54618-Q1 is expected to be capable of maintaining regulation without entering pulse-skipping mode
up to 2.35-MHz switching frequency under this operating condition. This result has been verified by bench
measurements to be accurate under extreme temperature corners on typical devices.
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TPS54618-Q1 High-Frequency (2.35 MHz) Operation
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