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Texas Instruments Using LMZ31710 for Higher Output Voltages Application notes
Application Report
SNVA749 – January 2016
Using LMZ31710 for Higher Output Voltages
....................................................................................................................... SVA - Simple Switcher
ABSTRACT
The LMZ31710 power module has been designed and characterized for operation at output voltages from
0.6 V to 5.5 V. However, several applications require higher voltages than the standard point-of-load
voltages that the LMZ31710 is designed for. Following the guidance in this application note, the
LMZ31710 can be used for output voltages up to 9.5 V. All data included in this application note has been
taken at VIN = PVIN = 12 V.
Typical Application
Figure 1 shows a typical schematic for operating LMZ31710 at higher output voltages. The schematic is
similar to the standard applications shown in the datasheet; with the exception of the feedback resistor
divider requires two external resistors (RUPPER & RSET) and SENSE+ is now left open. Refer to Table 1 for
resistor values and switching frequency ranges for several output voltages. Table 2 shows several
switching frequencies and the corresponding RRT resistor.
LMZ31710
VIN
VIN / PVIN
12 V
CIN1
100 F
PVIN
CIN2
47 F
CIN3
47 F
SENSE+
VOUT
7.5 V
VOUT
COUT1
47 F
CIN4
0.1 F
COUT2
47 F
RUPPER
10 k!
VADJ
SS/TR
CSS
4.7 nF
RT/CLK
STSEL AGND PGND
RSET
866 !
RRT
90.9 k!
Figure 1. Typical High VOUT Schematic
Operating Input Voltage
All data and recommended operating conditions in this application note apply to a 12-V input voltage. Both
the VIN and PVIN were connected togther for all tests. Operation at higher input voltage is possible;
however it will affect power dissipation, efficiency, ripple voltage and thermal performance. Additional
testing may be required to ensure a safe design.
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Setting the Output Voltage
To set the output voltage for a high VOUT application, two external feedback resistors (RUPPER & RSET) are
required and the SENSE+ pin should be left open. Table 1 lists several output voltages, the required
RUPPER & RSET resistors, and the allowable switching frequency range for that voltage. For output voltages
not listed in the table, the RSET value can be calculated using Equation 1, while RUPPER remains 10 kΩ.
The output voltage range tested for this application note is 5.5 V to 9.5 V; from an input voltage of 12 V.
Operation at output voltages greater than 9.5 V must be tested. In any case, the maximum output voltage
should not exceed 80% of the input voltage and the power dissipation must be less than 4.5 W.
Table 1. Voltage Setting Resistors and Frequency Range
VOUT
RUPPER
RSET
Switching Frequency Range
6.0 V
10 kΩ
1.1 kΩ
600 kHz - 1.2 MHz
6.5 V
10 kΩ
1.02 kΩ
600 kHz - 1.2 MHz
7.0 V
10 kΩ
931 Ω
600 kHz - 1.2 MHz
7.5 V
10 kΩ
866 Ω
600 kHz - 1.2 MHz
8.0 V
10 kΩ
806 Ω
600 kHz - 1.2 MHz
8.5 V
10 kΩ
759 Ω
600 kHz - 1.1 MHz
9.0 V
10 kΩ
715 Ω
600 kHz - 1.0 MHz
9.5 V
10 kΩ
673 Ω
600 kHz - 750 kHz
RSET =
10
(k )
  VOUT  

 − 1
  0.6  
(1)
Switching Frequency
When operating the LMZ31710 at increased output voltages, the minimum allowable switching frequency
is 600 kHz. The switching frequency can either be set using a frequency setting resistor (RRT), or by
applying an external clock. The switching frequency range for several output voltages is listed in Table 1.
Operating near the lower end of the frequency range will improve efficiency, while increasing the
frequency will reduce output voltage ripple. The switching frequency range applies to VIN = PVIN = 12 V.
Table 2. Switching Frequency Setting Resistor
Switching Frequency
600 kHz
750 kHz
900 kHz
1.0 MHz
1.2 MHz
RRT
124 kΩ
90.9 kΩ
69.8 kΩ
63.4 kΩ
48.7 kΩ
Input and Output Capacitance
When operating the LMZ31710 at higher output voltages, the amount of required input and output
capacitance is different from what is required in the datasheet.
The higher output voltage also means the input current will increase. To ensure a solid input voltage,
additional input capacitors may be required. The minimum input capacitance is two 47 µF ceramic
capacitors, plus an additional 100 µF, non-ceramic bulk capacitor is recommended.
To ensure stability over the higher output voltage range, the allowable output capacitance range is 94 µF
to 300 µF. The minimum required capacitance must include at least two 47 µF, 16 V ceramic capacitors.
Additional output capacitance can be either ceramic, low-ESR polymer type, or a combination of the two.
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Power Dissipation
A key concern when adjusting the output voltage higher than the range of the LMZ31710 is power
dissipation. Keeping the power dissipation of the device within the range of the device’s datasheet will
ensure safe thermal performance. The maximum power dissipation shown in the datasheet is 4.5 W.
When increasing the output voltage, the power dissipation must be ≤ 4.5 W to ensure a thermally efficient
design. Figure 2 shows the power dissipation for several output voltages at multiple switching frequencies.
Power dissipation can also be calculated based on efficiency. Figure 3 shows the efficiency for the same
output voltages and switching frequencies.
5.0
VOUT, fSW
Power Dissipation (W)
4.5
9.5 V, 600 kHz
9.5 V, 750 kHz
8.5 V, 600 kHz
8.5 V, 750 kHz
8.5 V, 1 MHz
7.5 V, 600 kHz
7.5 V, 750 kHz
7.5 V, 1 MHz
6.5 V, 600 kHz
6.5 V, 750 kHz
6.5 V, 1 MHz
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
1
2
3
4
5
6
7
8
9
Output Current (A)
10
C001
Figure 2. Power Dissipation
100
95
Efficiency (%)
90
VOUT, fSW
85
9.5 V, 600 kHz
9.5 V, 750 kHz
8.5 V, 600 kHz
8.5 V, 750 kHz
8.5 V, 1 MHz
7.5 V, 600 kHz
7.5 V, 750 kHz
7.5 V, 1 MHz
6.5 V, 600 kHz
6.5 V, 750 kHz
6.5 V, 1 MHz
80
75
70
65
60
0
2
4
6
8
Output Current (A)
10
C001
Figure 3. Efficiency
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Thermal Performance
The thermal performance of the device is shown in the Safe Operating Area graph, Figure 2. The Safe
Operating Area graph shows the operating conditions at which the LMZ31710 internal components are at
or below their thermal limits. This graph applies to 12 V input voltage, increasing the input voltage will
result in even more de-rating. The graphs are taken at natural convection, or 0 LFM. Increasing the airflow
with improve the thermal de-rating.
90
Ambient Temperature (ƒC)
80
70
60
50
VOUT, fSW
40
9.5 V, 750 kHz
8.5 V, 1 MHz
7.5 V, 1 MHz
6.5 V, 1 MHz
30
20
0
1
2
3
4
5
6
7
Output Current (A)
8
9
10
C004
Figure 4. Safe Operating Area
Conclusion
Operating the LMZ31710 at output voltages beyond what is characterized in the datasheet is possible by
following the guidance of this application note. Restrictions and operating conditions outline in this
document must be followed to ensure a safe design. Final application testing must be performed on any
design to ensure proper operation and behavior. Refer to the LMZ31710 datasheet for electrical
characteristics and other device specifications. The high voltage operation described in the document also
applies to LMZ31707 (7-A device) and LMZ31704 (4-A device).
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