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參數資料
| 型號： | TPS53851RHHT |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 穩壓器 |
| 英文描述： | SWITCHING CONTROLLER, PQCC36 |
| 封裝： | PLASTIC, VQFN-36 |
| 文件頁數： | 57/66頁 |
| 文件大小： | 1264K |
| 代理商： | TPS53851RHHT |
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SLUS985 – DECEMBER 2009
www.ti.com
6.5
LAYOUT CONSIDERATIONS
6.5.1
Power Stage
A synchronous BUCK power stage has two primary current loops – The input current loop which carries
high AC discontinuous current while the output current loop carries high DC continuous current. The input
current loop includes the input capacitors, the main switching MOSFET, the inductor, the output capacitors
and the ground path back to the input capacitors. To keep this loop as small as possible, it is generally
good practice to place some ceramic capacitance directly between the drain of the main switching
MOSFET and the source of the synchronous rectifier (SR) through a power ground plane directly under
the MOSFETs. The output current loop includes the SR MOSFET, the inductor, the output capacitors, and
the ground return between the output capacitors and the source of the SR MOSFET. As with the input
current loop, the ground return between the output capacitor ground and the source of the SR MOSFET
should be routed under the inductor and SR MOSFET to minimize the power loop area. The SW node
area should be as small as possible to reduce the parasitic capacitance and minimize the radiated
emissions. The gate drive loop impedance (HDRV-gate-source-SW and LDRV-gate-source- GND) should
be kept to as low as possible. The HDRV and LDRV connections should widen to 20mils as soon as
possible out from the device pin.
6.5.2
Device Peripheral
The TPS53851 provides separate signal ground (GND) and power ground (PGND) pins. It is required to
separate properly the circuit grounds. The return path for the pins associated with the power stage should
be through PGND. The other pins especially for those sensitive pins such as FB, RT and ILIM should be
through the low noise GND. The GND and PGND plane are suggested to be connected at the output
capacitor with single 20 mil trace. A minimum 0.1-
μF ceramic capacitor must be placed as close to the
VDD pin and GND as possible with at least 15-mil wide trace from the bypass capacitor to the GND. A
4.7-
μF ceramic capacitor should be placed as close to VREG pin and GND as possible. BP5 is the filtered
input from the VREG pin. A 4.7 -
resistor should be connected between VREG and BP5 and a 1-μF
ceramic capacitor should be connected from BP5 to GND. Both components should be as close to BP5
pin as possible. When DCR sensing method is applied, the sensing resistor is placed close to the SW
node. It is connected to the inductor with Kelvin connection. The sensing traces from the power stage to
the chip should be away from the switching components. The sensing capacitor should be placed very
close to the CS and CSRT pins. The frequency setting resistor should be placed as close to RT pin and
GND as possible. The VOUT and GSNS pins should be directly connected to the point of load where the
voltage regulation is required. A parallel pair of 10-mil traces connects the regulated voltage back to the
chip. They should be away from the switching components. The PowerPAD should be electrically
connected to GND.
6.5.3
PowerPAD Layout
The PowerPAD package provides low thermal impedance for heat removal from the device. The
PowerPAD derives its name and low thermal impedance from the large bonding pad on the bottom of
the device. The circuit board must have an area of solder-tinned-copper underneath the package. The
dimensions of this area depend on the size of the PowerPAD package.
Thermal vias connect this area to internal or external copper planes and should have a drill diameter
sufficiently small so that the via hole is effectively plugged when the barrel of the via is plated with copper.
This plug is needed to prevent wicking the solder away from the interface between the package body and
the solder-tinned area under the device during solder reflow. Drill diameters of 0,33 mm (13 mils) works
well when 1-oz copper is plated at the surface of the board while simultaneously plating the barrel of the
via. If the thermal vias are not plugged when the copper plating is performed, then a solder mask material
should be used to cap the vias with a diameter equal to the via diameter plus 0,1 mm minimum. This
capping prevents the solder from being wicked through the thermal vias and potentially creating a solder
void under the package. Refer to PowerPAD Thermally Enhanced Package for more information on the
PowerPAD package.
60
DESIGN EXAMPLES
Copyright 2009, Texas Instruments Incorporated
Product Folder Link(s): TPS53851
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