SC4525EEVB datasheet(17/23 Pages) SEMTECH

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SC4525E

Applications Information (Cont.)

Thermal Considerations

For the power transistor inside the SC4525E, the

conduction loss P

C, the switching loss PSW, and bootstrap

circuit loss P

BST, can be estimated as follows:

CESAT

BST

whereV

BST is the BST supply voltage and tS is the equivalent

switching time of the NPN transistor (see Table 4).

Table 4. Typical switching time

12V

12.5ns

15.3ns

18ns

24V

22ns

25ns

28ns

28V

25.3ns

28ns

31ns

Load Current

Input Voltage

In addition, the quiescent current loss is

2mA

The total power loss of the SC4525E is therefore

BST

TOTAL

The temperature rise of the SC4525E is the product of the

total power dissipation (see previous equation) and q

(36oC/W), which is the thermal impedance from junction

to ambient for the SOIC-8 EDP package.

It is not recommended to operate the SC4525E above

25oC junction temperature.

PCB Layout Considerations

In a step-down switching regulator, the input bypass

capacitor, the main power switch and the freewheeling

diode carry pulse currents (Figure 0). For jitter-free

operation,thesizeoftheloopformedbythesecomponents

should be minimized. Since the power switch is already

integrated within the SC4525E, connecting the anode of

the freewheeling diode close to the negative terminal of

the input bypass capacitor minimizes size of the switched

current loop. The input bypass capacitor should be placed

close to the IN pin. Shortening the traces of the SW and

BST nodes reduces the parasitic trace inductance at these

nodes. This not only reduces EMI but also decreases

switching voltage spikes at these nodes.

The exposed pad should be soldered to a large ground

plane as the ground copper acts as a heat sink for the

device. To ensure proper adhesion to the ground plane,

avoid using large vias directly under the device.

O UT

Figure 10 — Pulse Current Loop

NOTE: Heavy lines indicate the critical pulse current loop. The stray

inductance of this loop should be minimized.

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