SA57254-30GW Datasheet, PDF (15/18 Pages) NXP Semiconductors – CMOS switching regulator (PWM controlled)

Philips Semiconductors
CMOS switching regulator (PWM controlled)
Product data
SA57254-XX
Laying out the printed circuit board
The design of the printed circuit board (PCB) is critical to the proper
operation of all switching power supplies. Its design affects the
supply stability, radio frequency interference behavior and the
reliability of the converter.
Never use the autoroute feature of any PCB design program
because this will always produce traces that are too long and too
thin.
The input and output capacitors are the only source or sink of the
high frequency currents found in a switching power supply. All
connections to the switching power supply from the outside circuits
should be made to the input or output capacitor terminals (+ and –).
Internally, the layout should adhere to a “one-point” grounding
system, as shown in Figure 30.
V_IN
C_IN
L₀
SW
V_DD
FB
SA57254-XX
V_OUT
C_OUT
INPUT
GROUND
TO ONE POINT
GND
OUTPUT
GROUND
TO ONE POINT
SL01507
Figure 30. Grounding trace for converter.
The traces between the input and output capacitors and the
inductor, power switch and rectifier(s) should be as short and wide
as possible. This reduces the series resistance and inductance that
can be introduced by traces.
The guidelines for a PCB layout can be summarized as:
• The traces between the input and output capacitor to the inductor,
power switch and the rectifier should be made as short and as
wide as possible.
• Strictly adhere to the one-point wiring practices shown in
Figure 30.
• On a 2-sided board, do not run sensitive signals traces under the
AC voltage node.
• The IC (control) ground is terminated at the output capacitor’s
negative terminal.
Designing the PCB for effective heat dissipation
The maximum junction temperature is +125 °C, which should not be
exceeded under any operating conditions. Designing a PCB that
includes a heatsink system under the device is the key to cooler
operation of the circuit, and the long–term reliable operation of the
converter.
The major sources of heat within the converter are the power switch
inside the SA57254-XX, the resistive losses within the inductor, and
losses associated with the output rectifier. These losses can be
estimated by the following equations:
Power switch:
P_D(sw) ^ T_ON I_PK² R_DS(ON) f_SW
Eqn. (11)
Inductor:
P_D(L0) ^ I_pk²
R_winding
Eqn. (12)
Output rectifier:
P_D(rect) ^ I_OUT(Vfwd)
Eqn. (13)
The thermal resistance (R_th(j-a)) of the SA57254-XX is approximately
220 °C/W, assuming the device is soldered to a 2 oz. copper FR4
fiberglass circuit board, and that the minimum footprint was used
(copper just under the leads). A rule of thumb in PCB design is that
the thermal resistance can be reduced by 30% for each doubling of
the copper area close to the device. This effect diminishes for areas
greater than five times the minimum PCB footprint. If you take
advantage of this rule, thermal resistance can be reduced by using
wide copper lands when connecting to the leads of the major
power-producing parts. These PCB traces should almost fill the
areas surrounding the converter parts to conduct heat away from
the device. For demanding applications, additional heat dissipation
area can be created by placing a copper island on the opposite side
of the PCB from each wide trace and connecting it to the trace with
vias (plated thru holes).
The junction temperature can be estimated by Equation (14).
T_j ^ (P_D R_th(j-a)Ȁ) ) T_amb(max)
Eqn. (14)
Where:
P_D is the power dissipation (W).
R_th(j-a)′ is the effective thermal resistance with the additional
copper (°C/W).
T_amb is the highest local expected ambient temperature (°C).
2003 Nov 11
15