Texas Instruments | DRV3201 Boost Converter | Application notes | Texas Instruments DRV3201 Boost Converter Application notes

Texas Instruments DRV3201 Boost Converter Application notes
Application Report
SLVA741 – November 2015
DRV3201 Boost Converter
................................................................. Motor Drive Business Unit - Advanced Protection Motor Drivers
ABSTRACT
The DRV3201 boost converter is used to drive the external power MOSFETs. This type of converter
allows the DRV3201 to continue full operation of the external bridges down to a lower voltage than other
DC-DC converters. It is important to understand the requirements of the external components to power the
boost to ensure proper operation. This application report describes the boost converter for the DRV3201.
1
2
3
Contents
Boost Converter .............................................................................................................. 1
Application Circuit for Boost Converter ................................................................................... 3
Boost Converter Noise Reduction ......................................................................................... 4
List of Figures
1
Coil Current Waveforms in Steady State for Nominal, High and Low Battery Voltage ............................. 2
2
Boost Waveforms Showing Burst Pulsing Controlled by Hysteretic Comparator Levels ........................... 3
3
Recommended Application Circuit for Boost Converter ................................................................ 3
4
Boost Output and Current Sense O1 Output, Before and After Bypass Capacitor ................................. 4
5
High Current Path During Boost FET 'On' State ......................................................................... 5
6
High Current Path During Boost FET 'Off' State ......................................................................... 5
7
High Switching Transient Current Return Paths ......................................................................... 6
List of Tables
1
Boost Converter
The output current capability of the Boost Converter can be configured with the external Rshunt_boost
resistor to 0.1 V/Rshunt_boost (please note that this resistor needs to be able to conduct the boost
switching current). In this way, the output current capability can be dimensioned to the needed current
determined by the PWM switching frequency and the gate-charge of the external power FETs. TI
recommends choosing a coil having a current saturation level of at least 30% above the current limit level
set with the resistor Rshunt_boost .
The operation principle of the Boost Converter is based on a burst-mode fixed-frequency controller. During
the on-time, the internal low-side boost FET turned on until the current limit level is detected; the off-time
is calculated proportionally from a 2.5-MHz time-reference by sensing the supply voltage (VS) and the
output voltage (VBOOST). The formula for the calculated off-time is given in Equation 1, with ƒboost = 2.5
MHz.
VS
t off =
V BOOST ´ f BOOST
(1)
For steady-state, the current in the coil will look as illustrated in Figure 1.
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Boost Converter
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High battery
voltage at VS
Nominal battery
voltage at VS
Low battery
voltage at VS
IL
ILcurlim =
0.1V / Rboost_shunt
'Itoff =
(VBOOST-VS)*toff / L
'Iton =
VS*ton
/L
ton
ton
toff =
VS / (VBOOST * fBOOST)
ton+toff = 1 / fBOOST
toff =
VS / (VBOOST * fBOOST)
ton
toff =
VS / (VBOOST * fBOOST)
ton+toff = 1 / fBOOST
ton+toff = 1 / fBOOST
Figure 1. Coil Current Waveforms in Steady State for Nominal, High and Low Battery Voltage
From this figure, the ripple current and the boost output current can be calculated as follows:
IL ripple =
I BOOST =
æ
(VBOOST - VS) ´ VS
VS
VS ö
´ ç1 ÷=
L ´ f BOOST è
VBOOST ø L ´ f BOOST ´ V BOOST
VS
V BOOST
æ (VBOOST - VS) ´ VS
´ IL cur lim - 1 ´ ç
2 çL ´ f
BOOST ´ V BOOST
è
f BOOST = 2.5 MHz; (VBOOST - VS) = 15 V; IL cur lim
(2)
ö
÷
÷
ø
æ
0.1 V
=ç
ç R shunt _ boost
è
(3)
ö
÷
÷
ø
(4)
As Equation 3 shows, the boost output current capability for a given IL_curlim is the lowest for the
minimum supply voltage VS. So the boost output current capability needs to be dimensioned (by setting
IL_curlim with external Rshunt_boost) such that the needed output current (based on PWM frequency and
gate-charge of the external power FETs) can be delivered at the needed minimum supply voltage for the
application. Equation 5 gives IL_curlim as a function of IBOOST and VS:
IL cur lim = I BOOST ´
V BOOST
VS
æV
- VS ö
+ 1 ´ ç BOOST
÷
2
è L ´ f BOOST ø
(5)
For setting the IL_curlim, the minimum application supply needs to be used in this equation and IBOOST
according to Equation 5. The minimum application supply voltage which the DRV3201 can support is 4.75
V.
As shown by Equation 3, the boost output current capability increases for higher supply voltage VS. In
case the boost output current capability is dimensioned such that it can deliver the necessary output
current for the minimum supply voltage, it actually will deliver more current than needed for nominal supply
voltage. This will cause the boost voltage to increase. Therefore, a hysteretic comparator (low-level
VBOOST – VS = 14 V, high level VBOOST – VS = 16 V) determines starting/stopping the burst pulsing,
as Figure 2 illustrates.
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Application Circuit for Boost Converter
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VBOOST-VS
16V
14V
1) When VBOOST-VS>16V, boost FET kept on untill
current limit reached. No off-time calculated untill
VBOOST-VS<14V
2) When VBOOST-VS<14V,
new off-time calculated.
Boost FET turned on after off-time passed
IL
ILcurlim =
0.1VRboost_shunt
ton
toff =
VS / (VBOOST * fBOOST)
toff = VS / (VBOOST * fBOOST)
toff = VS / (VBOOST * fBOOST)
ton+toff = 1 / fBOOST
ton
toff = VS / (VBOOST * fBOOST)
ton+toff = 1 / fBOOST
Figure 2. Boost Waveforms Showing Burst Pulsing Controlled by Hysteretic Comparator Levels
2
Application Circuit for Boost Converter
The recommended application for the Boost Converter is given in Figure 3. For the best performance a
Schottky diode and a 22-µH coil is required. The current limit for the internal FET (and hence the
maximum current in the coil) can be adjusted and is set to 0.1 V/0.33 Ω = 303 mA in the recommended
application.
Battery Voltage
DRV3201
VS
Circuit
Blocks
C2
L1
SW
C1
D1
BOOST
C3
GNDLS_B
GNDA
R1
330m
AGND
Star Point
on PCB
Figure 3. Recommended Application Circuit for Boost Converter
All capacitors must be appropriately sized to handle the boost voltage. The following list shows the part
numbers for the primary components:
L1 = B82442A1223K000
D1 = SS28
C1 = 1 µF
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Boost Converter Noise Reduction
3
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Boost Converter Noise Reduction
In addition to the boost capacitor C1, two bypass capacitors, C2 and C3, are placed to reduce the ringing
effect from the converter switching. Figure 4 shows the effect of adding two 0.1-µF bypass capacitors to
the boost ripple. The red trace is the rising edge of the boost ripple while the converter is switching. The
green trace shows the current sense output O1 to illustrate the possible coupling on the system. The first
diagram is taken from our DRV3201EVM without any bypass components and shows the coupling from
the converter switching to the boost output and the O1 current sense output. The addition of the bypass
capacitors in the second diagram show a decrease in the transient spikes on the boost output, and show
no coupling on the current sense output O1.
Figure 4. Boost Output and Current Sense O1 Output, Before and After Bypass Capacitor
The layout for the boost converter is critical to the device performance for both regulation and noise
suppression. The most important consideration when laying out the boost converter is to keep all high
current loops small. Figure 5 and Figure 6 show the high current paths during the on and off states of the
boost MOSFET during regulation.
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Boost Converter Noise Reduction
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Battery Voltage
DRV3201
VS
Circuit
Blocks
C2
L1
SW
C1
D1
BOOST
C3
GNDLS_B
GNDA
R1
330 m
AGND
Star Point
on PCB
Figure 5. High Current Path During Boost FET 'On' State
Battery Voltage
DRV3201
VS
Circuit
Blocks
C2
L1
SW
C1
D1
BOOST
C3
GNDLS_B
GNDA
R1
330 m
AGND
Star Point
on PCB
Figure 6. High Current Path During Boost FET 'Off' State
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Boost Converter Noise Reduction
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In addition to these high current paths, the loops for high current transients must be kept small. Figure 7
illustrates the high current transient loops that exist when at the instant the boost FET is switched on or
off.
High Switching Transient
Current Return Paths
Battery Voltage
DRV3201
High dv/dt
VS
Circuit
Blocks
C2
L1
SW
C1
D1
BOOST
C3
GNDLS_B
GNDA
R1
330 m
Figure 7. High Switching Transient Current Return Paths
If required, an additional series RC snubber circuit can be added to the switch pin to further reduce noise
due to the effects of parasitic inductance and capacitance. For additional details on selection of snubber
components, refer to Minimizing Ringing at the Switch Node of a Boost Converter (SLVA255).
6
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