AN3392 datasheet(36/57 Pages) STMICROELECTRONICS

36 / 57 page

External component selection

AN3392

36/57

Doc ID 018749 Rev 1

10.12

Diode selection

The SPV1020 requires 3 Schottky diodes: D1, D2 and D3, as shown in Figure 24.

D3 (with C11) protects the SPV1020 supply by filtering system switching noise.

D3 should be chosen for low forward drop so that it doesn’t impact system efficiency.

Maximum forward current is according to the maximum current required by the SPV1020. A

safe choice is around 20 mA. Maximum voltage applied to D3 depends on the PV panel, and

does not exceed 45 V due to the maximum allowable SPV1020 input voltage.

D1 is the alternative path for current flow when the SPV1020 is down and Vin is higher than

VOUT.

D2 is a bypass diode. It turns on in the case of shaded cells and provides an alternative path

for the current flowing from other panels.

D1 and D2 are power Schottky diodes that must support both:

●

Forward current comparable with the maximum current provided by the PV cells.

Assuming a PV panel with 6” poly crystalline silicon solar cells, then the maximum

current is 9 A.

●

Maximum reverse voltage according to the output voltage partitioning. This should be

at least 45 V due to the voltage rating of the SPV1020.

Furthermore, the forward voltage and reverse current of D1 and D2 should be as low as

possible in order to minimize the impact on system efficiency.

10.13

Protection devices

The SPV1020 demo board uses a protection Transil™ D4 to trigger voltage spikes higher

than 45 V (AMR of the SPV102) on the VOUT pins.

This component must be chosen according to the following rules:

VBR > VOUT_max;

and

VCL

≤45 V.

The STEVAL-ISV009V1 uses D4 which has VBR = 37 V and VCL = 40 V.

10.14

Pole-zero compensation

The SPV1020 controls the whole system stability by an internal loop on VOUT_SNS.

The transfer function of the loop depends on both the output capacitor and load.

Even though the stability can be fine tuned by trimming R5 and C8 on pin PZ_OUT, as

shown in Figure 24, their suggested values (R5 = 1 k

Ω and C8 = 22 nF) guarantee stability

in most applications.

In order to increase system response to output voltage changes without causing overvoltage

threshold triggering, C8 can be decreased down to 2.2 nF.

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