Rev 1.5

Page 36 of 41

03/2018

PI352x-00

Input Filter case 2 — Inductive source and local, external

(i.e., electrolytic type):

In order to simplify the analysis in this case, the voltage source

within the PI352x-00 should be included in the external

electrolytic capacitance value for this purpose. The stability

criteria will be:

Equation 8 shows that if the aggregate ESR is too small – for

system will be under-damped and may even become destabilized.

As noted, an octave of design margin in satisfying Equation 7

should be considered the minimum. When applying an electrolytic

capacitor for input filter damping the ESR value must be chosen to

avoid loss of converter efficiency and excessive power dissipation

in the electrolytic capacitor.

VDR Bias Regulator

The VDR internal bias regulator is a ZVS switching regulator that

resides internal to the PI352x-00 SiP. It is intended primarily to

power the internal controller and driver circuitry. The power

capability of this regulator is sized for the PI352x-00, with

adequate reserve for the application it was intended for.

It may be used for as a pullup source for open collector

applications and for other very low power uses with the

following restrictions:

2. No direct connection is allowed. Any noise source that can

disturb the VDR voltage can also affect the internal controller

operation. A series impedance is required between the VDR pin

and any external circuitry.

3. All loads must be locally de-coupled using a 0.1µF ceramic

capacitor. This capacitor must be connected to the VDR output

through a series resistor no smaller than 1kΩ, which forms a

low-pass filter.

Additional System Design Considerations

1. Inductive loads: As with all power electronic applications,

consideration must be given to driving inductive loads that

may be exposed to a fault in the system which could result

in consequences beyond the scope of the power supply

primary protection mechanisms. An inductive load could be a

filter, fan motor or even excessively long cables. Consider an

instantaneous short circuit through an un-damped inductance

that occurs when the output capacitors are already at an

initial condition of fully charged. The only thing that limits the

current is the inductance of the short circuit and any series

resistance. Even if the power supply is off at the time of the

short circuit, the current could ramp up in the external inductor

and store considerable energy. The release of this energy will

result in considerable ringing, with the possibility of ringing

nodes connected to the output voltage below ground. The

system designer should plan for this by considering the use

of other external circuit protection such as load switches,

fuses, and transient voltage protectors. The inductive filters

should be critically damped to avoid excessive ringing or

damaging voltages. Adding a high current Schottky diode

from the output voltage to PGND close to the PI352x-00 is

recommended for these applications.

2. Low voltage operation: There is no isolation from an SELV

(Safety-Extra-Low-Voltage) power system. Powering low

voltage loads from input voltages as high as 60V may require

additional consideration to protect low voltage circuits from

excessive voltage in the event of a short circuit from input to

output. A fast TVS (transient voltage suppressor) gating an

external load switch is an example of such protection.

Table 5 — Recommended input and output capacitor components

Manufacturer

Part Number

Value

Description

Murata

GRM32EC80J107ME20

100µF

100µF 6.3V 1210 X6S

Murata

GRM32ER71A476KE15

47µF

47µF 10V 1210 X7R

Murata

GRM32ER72A225KA35

2.2µF

2.2µF 100V 1210 X7R

Murata

GRM32DR71E106MA12

10µF

10µF 25V 1210 X7R

Product

Load

Current

(A)

Ripple

Current

Ripple

Current

Ripple

(mVpp)

Ripple

(mVpp)

Load

Step

(% Rating)

(1A/µs)

Transient

Deviation

Excluding

Ripple

(mVpk)

Recovery

Time (µs)

PI3523

22

10 x

2.2µF

8 x 100µF

7.3

16.1

900

75

50 – 100

110

<80

PI3525

20

10 x

2.2µF

12 x 47µF

8.0

14

960

75

50 – 100

160

<80

PI3526

18

10 x

2.2µF

8 x 10µF

10.1

11

700

210

50 – 100

260

<80

Table 6 — Recommended input and output capacitor quantity and performance at nominal line, nominal trim.

(7)

r

L

line

C

< r

EQ_IN

(8)