lm5085sdx National Semiconductor Corporation, lm5085sdx Datasheet - Page 16

• Manufacturer Part Number: lm5085sdx
• Description: 75v Constant On-time Pfet Buck Switching Controller
• 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
LM5085 package is calculated using Equation 12 at the max-
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
imum input voltage. The Total Gate Charge for the Si7465
PFET is specified to be 40 nC (max) in its data sheet. There-
fore the total power dissipation within the LM5085 is calcu-
lated to be:
Using an MSOP-8EP package with a θ
a temperature rise of 34°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.
P
DISS
= 55V x ((40 nC x 300 kHz) + 1.4 mA) = 737 mW
JA
of 46°C/W produces
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