Reference Design Note

DNS002 V2.0

Roland Kratz - December 2018

Page 3/8

Copyright © Würth Elektronik eiSos GmbH & Co. KG

4. Circuit Description

In figure 3, C1 is the bulk capacitor of the input of the reference design. This capacitor decreases possible oscillations,

which may occur due to the mismatch between the line impedance and the “negative input resistance” of the power module.

C2, C3, C4 and C5 are the low impedance input capacitors for the power module. The switching current flows through these

capacitors when the power module’s high side FET conducts. It is recommended to choose all input capacitors of the same

type (order code) to prevent resonance between the capacitors with different impedances. The capacitor C5 is placed in a

way that minimizes the copper track length between the VIN pin of the power module and the capacitor and still maintaining

a short path to PGN. C6, C7 and C8 are the output capacitors of the power module. These are MLCC types with very low

ESR and assure a low output voltage ripple. It is recommended to choose all output capacitors to be the same type. The

capacitor C9 is used to decrease the transient response deviation of the output voltage and is an aluminum electrolytic

capacitor which offers a very high capacitance value. The resistor R26 sets the internal switching frequency to 1MHz. The

output resistor divider R4 and R11 sets the output voltage to 15.8V, slightly above the maximum Vout of 15V, controlled by

the external loop. Any current through R2 which passes also R11 will be subtracted from the current in R4 because the

regulation of the power module maintains the feedback node at 0.8V. This basic principle shows that the output voltage as

well as the output current can be set to any value between 0V and 15V and 0A to 2.5A respectively, by adjusting the current

through R2.

4.1 Output Voltage Regulation

The current measurement shunt resistor R3 is located between the ground of the regulation circuit and the output ground

drop of the current sense resistor R3, the output voltage is measured in addition at the capacitor C9 with the resistor divider

R1 and R32. The voltage across the resistor R3 is subtracted from the output voltage by the error amplifiers IC5-2 and IC5-

3. The result is the actual output voltage value referenced to analog ground which is passed to the error amplifier IC3-2, for

voltage regulation. The set value of the output voltage is adjusted by the potentiometer R29 and passed to the error amplifier

IC3-2, which regulates the output voltage by maintaining the non inverting input to the same voltage level as the inverting

input.

4.2 Output Current Regulation

The voltage across the current sense resistor R3 is proportional to the output current and referenced to the analog ground

via the layout. This actual value of the output current is passed to the error amplifier IC3-3 which maintains it to the set

value of the output current given by the potentiometer R30.

4.3 Interaction of Voltage and Current Regulation

When D1 conducts, indicating that the current regulation is active by IC3-3, the output voltage is always lower compared to

the value set by the potentiometer R29. This results in a lower voltage at the non inverting input of the error amplifier IC3-

2 compared to its inverting input. As a result the output voltage value of IC3-2 is decreased and blocked by the diode D3

and will not affect the active current regulation.

When D3 conducts, indicating that the voltage regulation is active by IC3-2, the output current is always lower compared to

the value set by the potentiometer R30. This results in a lower voltage at the non inverting input of the error amplifier IC3-

3 compared to its inverting input. As a result the output voltage value of IC3-3 is decreased and blocked by the diode D1

and will not affect the active voltage regulation.

4.4 Functions of the LEDs

If the current regulation is active, the red LED, D4, will glow. It is marked on the reference design board with “current

regulation”. The green LED D2 and the red LED D5, are based on the power good signal of the Power Module. If the

feedback voltage of the Power Module is within a small window around the internal reference voltage, the green LED, D2,

will glow. It is marked on the reference design board with “voltage regulation”. If the power good signal indicates a failure

the red LED, D4, will glow. It is marked on the reference design board with “fail”.