AN2559 datasheet(14/35 Pages) STMICROELECTRONICS

14 / 35 page

PM6680A block

AN2559

14/35

3.1

DC-DC converters based on the L5970AD

There are two converters based on the L5970AD on the System Supply board: the analog

output and VSYS output voltage. Figure 11 shows the arrangement of output voltages on

connector J18.

Figure 11.

Output voltages of L5970A parts

The L5970AD is a step-down monolithic power switching regulator with a switch current limit

of 1.5 A, capable of delivering more than 1 A of DC current to the load depending on the

application conditions. The output voltage can be set from 1.235 V to 35 V. The device uses

an internal P-channel D-MOS transistor (with a typical RDSON of 200 mΩ) as a switching

element to avoid the use of a bootstrap capacitor and to guarantee high efficiency. An

internal oscillator fixes the switching frequency at 500 kHz to minimize the size of external

components. Having a minimum input voltage of only 4.4 V, it is particularly suitable for 5 V

buses, found in all computer-related applications. Pulse-by-pulse current limiting with

internal frequency modulation offers effective constant current short circuit protection.

The schematic of both SMPS’s is displayed in Figure 12. As the schematic shows, designing

with the L5970AD is very simple. It consists of a power part, feedback and enable/disable

connectors. The power part contains an input capacitor (C2 or C8 - ceramic is

recommended), an inductor (L1 or L2), an output capacitor (C5 or C11) and a freewheeling

diode (D4 or D6). The feedback part consists of a voltage divider (R2, R3, R4 or R6, R7, R8)

and a compensation RC network (R1, C3, C4 or R5, C9, C10).

AN2559 Datasheet(PDF) 14 Page - STMicroelectronics