Datasheet

Design Procedures
2-5
Design Procedure
Assuming total switching time, t
r+f
, = 100 ns, a 55°C maximum ambient tem-
perature, and r
DS(ON)
adjustment factor (for high temperature) = 1.6, then:
P
D
2.5
2
(0.04 1.6) 0.7
0.5 5.5 2.5 0.1 10
6
275 10
3
0.41 W
The thermal impedance R
θJA
= 90°C/W for FR-4 with 2-oz. copper and a one-
inch-square pattern, thus:
T
J
T
A
R
JA
P
D
55 (90 0.41) 92°C
2.3.4 Rectifier
The catch rectifier conducts during the time interval when the MOSFET is off.
The 30WQ04 is a 3.3-A, 40-V rectifier in a D-Pak power surface-mount
package. The power dissipation is:
P
D
I
O
V
D
1 D
Min
2.5 0.6 0.68 1.02 W
2.3.5 Snubber Network
A snubber network is usually needed to suppress the ringing at the node where
the power switch drain, output inductor, and the rectifier connect. This is
usually a trial-and-error sequence of steps to optimize the network; but as a
starting point, select a snubber capacitor with a value that is 4–10 times larger
than the estimated capacitance of the catch rectifier. The 30WQ04 has a
capacitance of 110 pF, resulting in a snubber capacitor of 1000 pF. Then,
measuring a ringing time constant of 20 ns, R is:
R
20 10
–9
C
20 10
–9
1000 10
–12
20
A 22-Ω resistor is used in the design.
2.3.6 Controller Functions
The controller functions, oscillator frequency, soft-start, dead-time control,
short-circuit protection, and sense-divider network are discussed in this
section.
The oscillator frequency is set by selecting the resistance value from the graph
in Figure 6 of the TL5001 data sheet. For 275 kHz, a value of 30.1 k is
selected.
Dead-time control provides a minimum off-time for the power switch in each
cycle. Set this time by connecting a resistor between DTC and GND. For this
design, a maximum duty cycle of 100% is chosen. Then R is calculated as:
R
R
OSC
1.25 k D(V
0(100%)
– V
0(0%)
) V
0(0%)
(30.1 k 1.25 k )[1(1.4 – 0.6) 0.60] 44 k 47 k