LM34927SDX/NOPB [TI]

适用于隔离式直流/直流转换器的 7.5V 至 100V 宽输入电压、600mA 集成二次侧偏置稳压器 | NGU | 8 | -40 to 125;
LM34927SDX/NOPB
型号: LM34927SDX/NOPB
厂家: TEXAS INSTRUMENTS    TEXAS INSTRUMENTS
描述:

适用于隔离式直流/直流转换器的 7.5V 至 100V 宽输入电压、600mA 集成二次侧偏置稳压器 | NGU | 8 | -40 to 125

开关 控制器 开关式稳压器 开关式控制器 光电二极管 电源电路 转换器 开关式稳压器或控制器
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April 5, 2012  
LM34927  
Integrated Secondary Side Bias Regulator for Isolated DC-  
DC Converters  
General Description  
Features  
The LM34927 regulator features all of the functions needed  
to implement a low cost, efficient, isolated bias regulator. This  
high voltage regulator contains two 100V N-Channel MOS-  
FET switches - a high-side buck switch and a low-side syn-  
chronous switch. The Constant-on-time (COT) control  
scheme employed in the LM34927 requires no loop compen-  
sation and provides excellent transient response. The regu-  
lator operates with an on-time that is inversely proportional to  
the input voltage. This feature allows the operating frequency  
to remain relatively constant. An intelligent peak current limit  
is implemented with integrated sense circuit. Other features  
include a programmable input under voltage comparator to  
inhibit operation during low-voltage conditions. Protection  
features include thermal shutdown and VCC Undervoltage  
Lockout (UVLO). The LM34927 is offered in LLP-8 and  
PSOP-8 plastic packages.  
Wide 9V to 100V Input Range  
Integrated 100V, High and Low Side Switches  
No Schottky Required  
Constant On-Time Control  
No Loop Compensation Required  
Ultra-Fast Transient Response  
Nearly Constant Operating Frequency  
Intelligent Peak Current Limit  
Adjustable Output Voltage From 1.225V  
Precision 2% Feedback Reference  
Frequency Adjustable to 1MHz  
Adjustable Undervoltage Lockout (UVLO)  
Remote Shutdown  
Thermal Shutdown  
Packages  
LLP-8  
PSOP-8  
Applications  
Isolated Telecom Bias Supply  
Isolated Automotive and Industrial Electronics  
Typical Application  
30177901  
FIGURE 1. Typical Application Schematic  
© 2012 Texas Instruments Incorporated  
301779 SNVS799  
www.ti.com  
Connection Diagram  
30177903  
Top View (Connect Exposed Pad to RTN)  
30177902  
Top View (Connect Exposed Pad to RTN)  
Ordering Information  
Order Number  
Package Type  
PSOP-8  
Package Drawing  
MRA08A  
Supplied As  
LM34927MR  
LM34927SD  
1000 Units on Tape and Reel  
1000 Units on Tape and Reel  
LLP-8  
SDC08B  
Pin Descriptions  
Pin  
1
Name  
RTN  
VIN  
Description  
Application Information  
Ground  
Ground connection of the integrated circuit.  
Operating input range is 9V to 100V.  
2
Input Voltage  
3
UVLO  
Input Pin of Undervoltage Comparator  
Resistor divider from VIN to UVLO to GND programs  
the undervoltage detection threshold. An internal  
current source is enabled when UVLO is above  
1.225V to provide hysteresis. When UVLO pin is  
pulled below 0.66V externally, the parts goes in  
shutdown mode.  
4
RON  
On-Time Control  
Feedback  
A resistor between this pin and VIN sets the switch on-  
time as a function of VIN. Minimum recommended on-  
time is 100ns at max input voltage.  
5
6
FB  
This pin is connected to the inverting input of the  
internal regulation comparator. The regulation level is  
1.225V.  
VCC  
Output from the Internal High Voltage Series Pass The internal VCC regulator provides bias supply for the  
Regulator. Regulated at 7.6V.  
gate drivers and other internal circuitry. A 1.0μF  
decoupling capacitor is recommended.  
7
8
BST  
Bootstrap Capacitor  
An external capacitor is required between the BST  
and SW pins (0.01μF ceramic). The BST pin capacitor  
is charged by the VCC regulator through an internal  
diode when the SW pin is low.  
SW  
EP  
Switching Node  
Exposed Pad  
Power switching node. Connect to the output inductor  
and bootstrap capacitor.  
Exposed pad must be connected to RTN pin. Connect  
to system ground plane on application board for  
reduced thermal resistance.  
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2
FB to RTN  
-0.3V to 5V  
Absolute Maximum Ratings (Note 1)  
ESD Rating (Human Body Model(Note 2kV  
5)  
Lead Temperature (Note 2)  
If Military/Aerospace specified devices are required,  
please contact the Texas Instruments Sales Office/  
Distributors for availability and specifications.  
200°C  
-55°C to +150°C  
Storage Temperature Range  
VIN, UVLO to RTN  
SW to RTN  
-0.3V to 100V  
-1.5V to VIN +0.3V  
100V  
Operating Ratings (Note 1)  
VIN Voltage  
BST to VCC  
9V to 100V  
−40°C to +125°C  
BST to SW  
13V  
Operating Junction Temperature  
RON to RTN  
VCC to RTN  
-0.3V to 100V  
-0.3V to 13V  
Electrical Characteristics  
Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction Temper-  
ature range. VIN = 48V, unless otherwise stated. See (Note 3).  
Symbol  
VCC Supply  
VCC Reg  
Parameter  
Conditions  
Min  
Typ  
7.6  
4.5  
Max  
8.55  
4.9  
Units  
VCC Regulator Output  
VCC Current Limit  
VIN = 48V, ICC = 20mA  
6.25  
26  
V
mA  
V
VIN = 48V(Note 4)  
VCC UVLO Threshold (VCC  
increasing)  
4.15  
VCC UVLO Hysteresis  
300  
2.3  
mV  
V
VCC Drop Out Voltage  
IIN Operating Current  
IIN Shutdown Current  
VIN = 8V, ICC = 20mA  
Non-Switching, FB = 3V  
UVLO = 0V  
1.75  
50  
mA  
µA  
225  
Under-Voltage Sensing Function  
UV Threshold  
UV Rising  
1.19  
-10  
1.225  
-20  
1.26  
-29  
V
µA  
V
UV Hysteresis Input Current  
UV = 2.5V  
Remote Shutdown Threshold  
Remote Shutdown Hysteresis  
Voltage at UVLO Falling  
0.32  
0.66  
110  
mV  
Regulation and Over-Voltage Comparators  
FB Regulation Level  
Internal Reference Trip  
Point for Switch ON  
1.2  
1.225  
1.25  
V
FB Overvoltage Threshold  
FB Bias Current  
Trip Point for Switch OFF  
1.62  
60  
V
nA  
Switch Characteristics  
Buck Switch RDS(ON)  
ITEST = 200mA, BST-SW =  
0.8  
1.8  
7V  
Synchronous RDS(ON)  
Gate Drive UVLO  
Gate Drive UVLO Hysteresis  
Minimum Off-Time  
Minimum Off-Timer  
On Time Generator  
TON Test 1  
ITEST = 200mA  
0.45  
3
1
V
VBST − VSW Rising  
2.4  
3.6  
260  
mV  
FB = 0V  
144  
ns  
VIN = 32V, RON = 100k  
VIN = 48V, RON = 100k  
VIN = 75V, RON = 250k  
VIN = 10V, RON = 250k  
270  
188  
350  
250  
460  
336  
ns  
ns  
ns  
ns  
TON Test 2  
TON Test 3  
250  
370  
500  
TON Test 4  
1880  
3200  
4425  
3
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Symbol  
Parameter  
Conditions  
Min  
0.7  
Typ  
Max  
1.3  
Units  
Current Limit  
Current Limit Threshold  
1.02  
150  
12  
A
Current Limit Response Time  
OFF-Time Generator (Test 1)  
OFF-Time Generator (Test 2)  
Time to Switch Off  
FB = 0.1V, VIN = 48V  
FB = 1.0V, VIN = 48V  
ns  
µs  
µs  
2.5  
Thermal Shutdown  
Tsd Thermal Shutdown Temp.  
Thermal Shutdown Hysteresis  
Thermal Resistance  
Junction to Ambient  
165  
20  
°C  
°C  
PSOP-8  
LLP-8  
40  
40  
°C/W  
°C/W  
θJA  
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the  
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. The RTN pin is the GND reference  
electrically connected to the substrate.  
Note 2: For detailed information on soldering plastic PSOP package, refer to the Packaging Data Book available from National Semiconductor Corporation. Max  
solder time not to exceed 4 seconds.  
Note 3: All limits are guaranteed by design. All electrical characteristics having room temperature limits are tested during production at TA = 25°C. All hot and  
cold limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control.  
Note 4: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading.  
Note 5: The human body model is a 100pF capacitor discharged through a 1.5kresistor into each pin.  
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Typical Performance Characteristics  
Efficiency at 750kHz, VOUT1 = 10V  
VCC vs VIN  
30177905  
30177904  
VCC vs ICC  
ICC vs External VCC  
30177906  
30177907  
TON vs VIN and RON  
TOFF (ILIM) vs VFB and VIN  
30177909  
30177908  
5
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IIN vs VIN (Operating, Non Switching)  
IIN vs VIN (Shutdown)  
30177910  
30177911  
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Block Diagram  
30177913  
FIGURE 2. Functional Block Diagram  
7
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Functional Description  
The LM34927 step-down switching regulator features all the  
functions needed to implement a low cost, efficient, isolated  
bias supply. This high voltage regulator contains 100V, N-  
channel buck and synchronous switches, is easy to imple-  
ment, and is provided in thermally enhanced PSOP-8 and  
LLP-8 packages. The regulator operation is based on a con-  
stant on-time control scheme using an on-time inversely pro-  
portional to VIN. This control scheme does not require loop  
compensation. Current limit is implemented with forced off-  
time inversely proportional to VOUT. This scheme ensures  
short circuit protection while providing minimum foldback. The  
simplified block diagram of the LM34927 is shown in Figure  
2.  
The output voltage (VOUT) is set by two external resistors  
(RFB1, RFB2). The regulated output voltage is calculated as  
follows:  
The LM34927 can be applied in numerous applications to ef-  
ficiently regulate down higher voltages. This regulator is well  
suited for 48V telecom and 42V automotive power bus  
ranges. Protection features include: thermal shutdown, Un-  
dervoltage Lockout, minimum forced off-time, and an intelli-  
gent current limit.  
This regulator regulates the output voltage based on ripple  
voltage at the feedback input, requiring a minimum amount of  
ESR for the output capacitor (COUT). A minimum of 25mV of  
ripple voltage at the feedback pin (FB) is required for the  
LM34927. In cases where the capacitor ESR is too small, ad-  
ditional series resistance may be required (RC in Figure 3 Low  
Ripple Output Configuration).  
Control Overview  
For applications where lower output voltage ripple is required  
the output can be taken directly from a low ESR output ca-  
pacitor, as shown in Figure 3 Low Ripple Output Configura-  
tion. However, RC slightly degrades the load regulation.  
The LM34927 regulator employs a control principle based on  
a comparator and a one-shot on-timer, with the output voltage  
feedback (FB) compared to an internal reference (1.225V). If  
the FB voltage is below the reference the internal buck switch  
is switched on for the one-shot timer period, which is a func-  
tion of the input voltage and the programming resistor (RT).  
Following the on-time the switch remains off until the FB volt-  
age falls below the reference, and the forced minimum off-  
time has expired. When the FB pin voltage falls below the  
reference and the off-time one-shot period expires, the buck  
switch is then turned on for another on-time one-shot period.  
This will continue until regulation is achieved and the FB volt-  
age is approximately equal to 1.225V (typ).  
VCC Regulator  
The LM34927 contains an internal high voltage linear regu-  
lator with a nominal output of 7.6V. The input pin (VIN) can be  
connected directly to the line voltages up to 100V. The VCC  
regulator is internally current limited to 30mA. The regulator  
sources current into the external capacitor at VCC. This regu-  
lator supplies current to internal circuit blocks including the  
synchronous MOSFET driver and the logic circuits. When the  
voltage on the VCC pin reaches the Undervoltage Lockout  
threshold of 4.5V, the IC is enabled.  
In a synchronous buck converter, the low side (sync) FET is  
‘on’ when the high side (buck) FET is ‘off’. The inductor current  
ramps up when the high side switch is ‘on’ and ramps down  
when the high side switch is ‘off’. There is no diode emulation  
feature in this IC, and therefore, the inductor current may  
ramp in the negative direction at light load. This causes the  
converter to operate in continuous conduction mode (CCM)  
regardless of the output loading. The operating frequency re-  
mains relatively constant with load and line variations. The  
operating frequency can be calculated as follows:  
The VCC regulator contains an internal diode connection to  
the BST pin to replenish the charge in the gate drive boot  
capacitor when SW pin is low.  
At high input voltages, the power dissipated in the high volt-  
age regulator is significant and can limit the overall achievable  
output power. As an example, with the input at 48V and  
switching at high frequency, the VCC regulator may supply up  
to 7mA of current resulting in 48V x 7mA = 336mW of power  
dissipation. If the VCC voltage is driven externally by an alter-  
nate voltage source, between 8V and 14V, the internal regu-  
lator is disabled. This reduces the power dissipation in the IC.  
30177917  
FIGURE 3. Low Ripple Output Configuration  
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the input voltage and/or the output load changes suddenly.  
The high side switch will not turn on again until the voltage at  
FB falls below 1.225V.  
Regulation Comparator  
The feedback voltage at FB is compared to an internal 1.225V  
reference. In normal operation, when the output voltage is in  
regulation, an on-time period is initiated when the voltage at  
FB falls below 1.225V. The high side switch will stay on for  
the on-time, causing the FB voltage to rise above 1.225V. Af-  
ter the on-time period, the high side switch will stay off until  
the FB voltage again falls below 1.225V. During start-up, the  
FB voltage will be below 1.225V at the end of each on-time  
causing the high side switch to turn on immediately after the  
minimum forced off-time of 144ns. The high side switch can  
be turned off before the on-time is over, if peak current in the  
inductor reaches the current limit threshold.  
On-Time Generator  
The on-time for the LM34927 is determined by the RON resis-  
tor, and is inversely proportional to the input voltage (VIN),  
resulting in a nearly constant frequency as VIN is varied over  
its range. The on-time equation for the LM34927 is:  
See Figure 4 below. RON should be selected for a minimum  
on-time (at maximum VIN) greater than 100ns, for proper op-  
eration. This requirement limits the maximum frequency for  
each application.  
Overvoltage Comparator  
The feedback voltage at FB is compared to an internal 1.62V  
reference. If the voltage at FB rises above 1.62V the on-time  
pulse is immediately terminated. This condition can occur if  
30177908  
FIGURE 4. TON vs VIN and RON  
The current limit protection feature is peak limited, the maxi-  
mum average output will be less than the peak.  
Current Limit  
The LM34927 contains an intelligent current limit off-timer. If  
the current in the buck switch exceeds 1.02A the present cy-  
cle is immediately terminated, and a non-resetable off-timer  
is initiated. The length of off-time is controlled by the FB volt-  
age and the input voltage VIN. As an example, when FB = 0V  
and VIN = 48V, a maximum off-time is set to 16μs. This con-  
dition occurs when the output is shorted, and during the initial  
part of start-up. This amount of time ensures safe short circuit  
operation even up to the maximum input voltage of 100V.  
N-Channel Buck Switch and Driver  
The LM34927 integrates an N-Channel Buck switch and as-  
sociated floating high voltage gate driver. The gate driver  
circuit works in conjunction with an external bootstrap capac-  
itor and an internal high voltage diode. A 0.01uF ceramic  
capacitor connected between the BST pin and SW pin pro-  
vides the voltage to the driver during the on-time. During each  
off-time, the SW pin is at approximately 0V, and the bootstrap  
capacitor charges from VCC through the internal diode. The  
minimum off-timer, set to 144ns, ensures a minimum time  
each cycle to recharge the bootstrap capacitor.  
In cases of overload where the FB voltage is above zero volts  
(not a short circuit) the current limit off-time is reduced. Re-  
ducing the off-time during less severe overloads reduces the  
amount of foldback, recovery time, and start-up time. The off-  
time is calculated from the following equation:  
Synchronous Rectifier  
The LM34927 provides an internal synchronous N-Channel  
MOSFET rectifier. This MOSFET provides a path for the in-  
9
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ductor current to flow when the high-side MOSFET is turned  
off.  
the set-point divider. When the UVLO threshold is exceeded,  
the current source is activated to quickly raise the voltage at  
the UVLO pin. The hysteresis is equal to the value of this cur-  
The synchronous rectifier has no diode emulation mode, and  
is designed to keep the regulator in continuous conduction  
mode even during light loads which would otherwise result in  
discontinuous operation. This feature specifically allows the  
user to design a secondary regulator using a transformer  
winding off the main inductor to generate the alternate regu-  
lated output voltage.  
rent times the resistance RUV2  
.
UVLO  
VCC  
Mode  
Description  
<0.66V  
Shutdown VCC regulator  
disabled.  
Switcher disabled.  
0.66V –  
1.225V  
Standby VCC regulator  
enabled.  
Under Voltage Detector  
The LM34927 contains a dual level Undervoltage Lockout  
(UVLO) circuit. When the UVLO pin voltage is below 0.66V,  
the controller is in a low current shutdown mode. When the  
UVLO pin voltage is greater than 0.66V but less than 1.225V,  
the controller is in standby mode. In standby mode the VCC  
bias regulator is active while the regulator output is disabled.  
When the VCC pin exceeds the VCC undervoltage thresholds  
and the UVLO pin voltage is greater than 1.225V, normal op-  
eration begins. An external set-point voltage divider from  
VIN to GND can be used to set the minimum operating voltage  
of the regulator.  
Switcher disabled.  
>1.225V  
VCC < 4.5V Standby VCC regulator  
enabled.  
Switcher disabled.  
VCC > 4.5V Operating VCC enabled.  
Switcher enabled.  
If the UVLO pin is wired directly to the VIN pin, the regulator  
will begin operation once the VCC undervoltage is satisfied.  
UVLO hysteresis is accomplished with an internal 20μA cur-  
rent source that is switched on or off into the impedance of  
30177921  
FIGURE 5. UVLO Resistor Setting  
When activated, typically at 165°C, the controller is forced into  
a low power reset state, disabling the buck switch and the  
VCC regulator. This feature prevents catastrophic failures from  
accidental device overheating. When the junction tempera-  
ture reduces below 145°C (typical hysteresis = 20°C), the  
VCC regulator is enabled, and normal operation is resumed.  
Thermal Protection  
The LM34927 should be operated so the junction temperature  
does not exceed 150°C during normal operation. An internal  
Thermal Shutdown circuit is provided to protect the LM34927  
in the event of a higher than normal junction temperature.  
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Application Information  
TYPICAL ISOLATED BIAS APPLICATION SCHEMATIC  
A typical isolated bias supply application is shown in Figure  
6 below. Inductor (L) in a typical buck circuit is replaced with  
a coupled inductor (X1). A diode (D1) is used to rectify the  
voltage on the secondary output. The nominal voltage at the  
secondary output (VOUT2) is given by:  
where VF is the forward voltage drop of D1, and NP, NS are  
the number of turns on the primary and secondary of coupled  
inductor X1. For output voltage (VOUT1) above the maximum  
VCC (8.3V), the VCC pin can be diode connected to VOUT1 for  
higher efficiency and low dissipation in the IC.  
30177922  
FIGURE 6. Typical Isolated Application Schematic  
3W ISOLATED BIAS APPLICATION SCHEMATIC  
A complete 3W bias supply for isolated bias supply applica-  
tion is shown in Figure 7 below.  
30177933  
FIGURE 7. A 3W Isolated Application Schematic  
11  
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LOWEST PART COUNT ISOLATED APPLICATION  
SCHEMATIC  
in this configuration. If primary loading is required a diode will  
be required between VOUT primary and VCC  
.
A low part count schematic for isolated bias application is  
shown in Figure 8 below. The primary should not be loaded  
30177941  
FIGURE 8. Lowest Part Count Isolated Application Schematic  
RIPPLE CONFIGURATION  
The capacitive ripple is not in phase with the inductor current.  
As a result of this, the capacitive ripple does not decrease  
monotonically during the off-time. The resistive ripple is in  
phase with the inductor current and decreases monotonically  
during off-time. The resistive ripple must exceed the capaci-  
tive ripple at the output node (VOUT) for stable operation. If this  
condition is not satisfied, unstable switching behavior is ob-  
served in COT converters, with multiple on-time bursts in  
close succession followed by a long off-time.  
LM34927 uses Constant-On-Time (COT) control scheme, in  
which the on-time is terminated by an on-timer, and the off-  
time is terminated by the feedback voltage (VFB) falling below  
the reference voltage (VREF). Therefore, for stable operation,  
the feedback voltage must decrease monotonically, in phase  
with the inductor current during the off-time. Furthermore this  
change in feedback voltage (ΔVFB) during off-time must be  
large enough to suppress any noise component present at the  
feedback node.  
Type 3 ripple method uses Rr and Cr and the switch node  
(SW) voltage to generate a triangular ramp. This triangular  
ramp is ac coupled using Cac to the feedback node (FB). Since  
this circuit does not use the output voltage ripple, it is ideally  
suited for applications where low output voltage ripple is re-  
quired. See application note AN-1481 for more details for  
each ripple generation method.  
Table 1 shows three different methods for generating appro-  
priate voltage ripple at the feedback node. Type 1 and Type  
2 ripple circuits couple the ripple at the output of the converter  
to the feedback node (FB). The output voltage ripple has two  
components:  
1. Capacitive ripple caused by the inductor current ripple  
charging/discharging the output capacitor.  
2. Resistive ripple caused by the inductor current ripple  
flowing through the ESR of the output capacitor.  
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Type 1  
Type 2  
Type 3  
Lowest Cost Configuration  
Reduced Ripple Configuration  
Minimum Ripple Configuration  
13  
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side gate drivers. These two capacitors should also be  
placed as close to the IC as possible, and the connecting  
trace lengths and loop area should be minimized (See  
Figure 9 Placement of Bypass Capacitors).  
Layout Recommendation  
A proper layout is essential for optimum performance of the  
circuit. In particular, the following guidelines should be ob-  
served:  
3. The Feedback trace carries the output voltage  
information and a small ripple component that is  
necessary for proper operation of LM34927. Therefore  
care should be taken while routing the feedback trace so  
avoid coupling any noise to this pin. In particular,  
feedback trace should not run close to magnetic  
components, or parallel to any other switching trace.  
1. CIN: The loop consisting of input capacitor (CIN), VIN pin,  
and RTN pin carries switching currents. Therefore the  
input capacitor should be placed close to the IC, directly  
across VIN and RTN pins and the connections to these  
two pins should be direct to minimize the loop area. In  
general it is not possible to accommodate all of input  
capacitance near the IC. A good practice is to use a  
0.1μF or 0.47μF capacitor directly across the VIN and  
RTN pins close to the IC, and the remaining bulk  
capacitor as close as possible (Refer to Figure 9  
Placement of Bypass Capacitors).  
4. SW trace: SW node switches rapidly between VIN and  
GND every cycle and is therefore a possible source of  
noise. SW node area should be minimized. In particular  
SW node should not be inadvertently connected to a  
copper plane or pour.  
2. CVCC and CBST: The VCC and bootstrap (BST) bypass  
capacitors supply switching currents to the high and low  
30177940  
FIGURE 9. Placement of Bypass Capacitors  
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Physical Dimensions inches (millimeters) unless otherwise noted  
PSOP–8 Outline Drawing  
NS Package Number MRA08A  
LLP-8 Outline Drawing (Dimensions in mm)  
NS Package Number SDC08B  
15  
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