lm25085asdx National Semiconductor Corporation, lm25085asdx Datasheet - Page 16

• Manufacturer Part Number: lm25085asdx
• Description: 42v Constant On-time Pfet Buck Switching Controller With 0.9v Reference
• Manufacturer: National Semiconductor Corporation

www.national.com
C
vides not only noise filtering and stability for the VCC regula-
tor, but also provides the surge current for the PFET gate
drive. The typical recommended value for C
good quality, low ESR, ceramic capacitor is recommended.
C
VCC pins. If the selected PFET has a Total Gate Charge
specification of 100 nC or larger, or if the circuit is required to
operate at input voltages below 7 volts, a larger capacitor may
be required. The maximum recommended value for C
1 µF.
IC Power Dissipation: The maximum power dissipated in the
LM25085A package is calculated using Equation 12 at the
maximum input voltage. The Total Gate Charge for the
VCC
VCC
: The capacitor at the VCC pin (from VIN to VCC) pro-
must be located as close as possible to the VIN and
VCC
is 0.47 µF. A
FIGURE 4. Example Circuit
VCC
is
16
Si7465 PFET is specified to be 40 nC (max) in its data sheet.
Therefore the total power dissipation within the LM25085A is
calculated to be:
Using an MSOP-8EP package with a θ
a temperature rise of 10°C from junction to ambient.
Final Design Example Circuit
The final circuit is shown in Figure 4, and its performance is
presented in Figure 5 through Figure 8. The measured effi-
ciencies shown in Figure 5 are typical for a buck converter
producing a low output voltage (1V).
P
DISS
= 24V x ((40 nC x 200 kHz) + 1.25 mA) = 222 mW
JA
of 46°C/W produces
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