Datasheet

TPS62736
TPS62737
SLVSBO4B –OCTOBER 2012–REVISED JULY 2013 www.ti.com
The minor switching frequency is of concern when choosing the inductor. This maximum switching frequency is
1 MHz. The major switching frequency dictates the voltage ripple frequency. Figure 27 and Figure 28 show the
major switching frequency versus load current and input voltage for the TPS62736, respectively. Figure 61 and
Figure 62 show the major switching frequency versus load current and input voltage for the TPS62737,
respectively.
Inductor Selection
The internal control circuitry is designed to control the switching behavior with a nominal inductance of 10 µH ±
20%. The inductor's saturation current should be at least 25% higher than the maximum cycle-by-cycle current
limit per the electrical specs table (I
LIM
) in order to account for load transients. Since this device is a hysteretic
controller, it is a naturally stable system (single order transfer function). However, the smaller the inductor value
is, the faster the switching currents are. The speed of the peak current detect circuit sets the TPS62736
inductor's lower bound to 4.7 µH. When using a 4.7 µH, the peak inductor current will increase when compared
to that of a 10 µH inductor. The steady-state operation with a 4.7 µH inductor with a 50 mA load for the
TPS62736 is shown in Figure 33.
A list of inductors recommended for this device is shown in Table 2.
Table 2.
Inductance (µH) Dimensions (mm) Part Number Manufacturer
10 2.0 x 2.5 x 1.2 DFE252012C-H-100M Toko
10 4.0x4.0x1.7 LPS4018-103M Coilcraft
4.7 (TPS62736 only) 2.0 x 2.5 x 1.2 DFE252012R-H-4R7M Toko
Output Capacitor Selection
The output capacitor is chosen based on transient response behavior and ripple magnitude. The lower the
capacitor value, the larger the ripple will become and the larger the droop will be in the case of a transient
response. It is recommended to use at least a 22 µF output capacitor for most applications.
Input Capacitor Selection
The bulk input capacitance is recommended to be a minimum of 4.7 µF ± 20% for the TPS62736 and 22 µF ±
20% for the TPS62737. This bulk capacitance is used to suppress the lower frequency transients produced by
the switching converter. There is no upper bound to the input bulk capacitance. In addition, a high frequency
bypass capacitor of 0.1 µF is recommended in parallel with the bulk capacitor. The high frequency bypass is
used to suppress the high frequency transients produced by the switching converter.
Layout and PCB Assembly Considerations
To minimize switching noise generation, the step-down converter (buck) power stage external components must
be carefully placed. The most critical external component for a buck power stage is its input capacitor. The bulk
input capacitor (C
IN1
) and high frequency decoupling capacitor (C
IN2
) must be placed as close as possible
between the power stage input (IN pin 1) and ground (VSS pin 12). Next, the inductor (L1) must be placed as
close as possible beween the switching node (SW pin 13) and the output voltage (OUT pin 11). Finally, the
output capacitor (C
OUT
) should be placed as close as possible between the output voltage (OUT pin 11) and
GND (VSS pin 12). In the diagram below, the input and output capacitor grounds are connected to VSS pin 12
through vias to the PCB's bottom layer ground plane.
To minimize noise pickup by the high impedance voltage setting nodes (VIN_OK_SET pin 8 and VOUT_SET pin
9), the external resistors (R1, R2 and R3) should be placed so that the traces connecting the midpoints of the
string are as short as possible. In the diagram below, the connection to VOUT_SET is by a bottom layer trace.
The remaining pins are either NC pins, that should be connected to the PowerPAD ™ as shown below, or digital
signals with minimal layout restrictions.
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