Datasheet

TPS61045
www.ti.com SLVS440B –JANUARY 2003–REVISED MARCH 2009
INPUT CAPACITOR SELECTION
The input capacitor (C1) filters the high frequency noise to the control circuit and must be directly connected to
the input pin (VIN) of the device. The capacitor (C2) connected to the L pin of the device is the input capacitor for
the power stage.
The main purpose of the capacitor (C2), that is connected directly to the L pin, is to smooth the inductor current.
A larger capacitor reduces the inductor ripple current present at the L pin. The smaller the ripple current at the L
pin, the higher the efficiency of the converter. If a sufficiently large capacitor is used, the input switch must carry
only the dc current, filtered by the capacitor (C2), and not the high switching currents of the converter. A 4.7 mF
or 10-mF ceramic capacitor (C2) is sufficient for most applications. For better filtering, this value can be increased
without limit. See Table 4 and the TYPICAL APPLICATIONS for input capacitor recommendations.
Table 4. Possible Input and Output Capacitor Selection
CAPACITOR VOLTAGE RATING COMPONENT SUPPLIER COMMENTS
4.7 F/X5R/0805 6.3 V Tayo Yuden JMK212BY475MG C
I
/C
O
10 mF/X5R/0805 6.3 V Tayo Yuden JMK212BJ106MG C
I
/C
O
1 mF/X7R/1206 25 V Tayo Yuden TMK316BJ105KL C
O
1 mF/X7R/1206 35 V Tayo Yuden GMK316BJ105KL C
O
4.7 mF/X5R/1210 25 V Tayo Yuden TMK325BJ475MG C
O
DIODE SELECTION
To achieve high efficiency a Schottky diode must be used. The current rating of the diode must meet the peak
current rating of the converter as it is calculated in the peak current control section. Use the maximum value for
I
(LIM)
(450mA) for this calculation. See Table 5 and the TYPICAL APPLICATIONS for the selection of the
Schottky diode.
Table 5. Possible Schottky Diode Selection
COMPONENT SUPPLIER REVERSE VOLTAGE
ON Semiconductor MBR0530 30 V
ON Semiconductor MBR0520 20 V
ON Semiconductor MBRM120L 20 V
Toshiba CRS02 30 V
Zetex ZHZS400 40 V
LAYOUT CONSIDERATIONS
As for all switching power supplies the layout is an important step in the design, especially at high peak currents
and switching frequencies. If the layout is not carefully implemented, the regulator can show noise problems and
duty cycle jitter.
The inductor and diode must be placed as close as possible to the switch pin (SW) to minimize noise coupling
into other circuits. Since the feedback pin and network is a high impedance circuit, the feedback network must be
routed away from the inductor. Also the input capacitor must be placed as close as possible to the input pin for
good input-voltage filtering. If space is critical and the described can not be followed completely, the close
placement of the diode to the switch pin (SW) must be given the highest priority.
THERMAL CONSIDERATIONS
The TPS61045 is available in a thermally enhanced QFN package. The package includes a thermal pad,
improving the thermal capabilities of the package. See QFN/SON PCB attachment application note (SLUA271).
The thermal resistance junction to ambient (R
ΘJA
) of the QFN package depends on the PCB layout. By using
thermal vias and wide PCB, traces improve thermal resistance (R
ΘJA
). Under normal operation conditions no
PCB vias are required for the thermal pad. However, the thermal pad must be soldered to the PCB.
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