7

Active clamp

Active clamp is also known as fast demagnetization of inductive loads or fast current decay. When a low-side

driver turns off an inductance, an overvoltage on load is detected. If VZ pins are directly shorted to the supply rail

(see fig 7 Application circuit ( fast decay disabled by Vz shorted to supply rail ) . ) then the fast current decay is

disabled: the inductive load is demagnetized slowly and according to the forward voltage of the integrated clamp

diodes (VFCD). The figure below shows the typical waveforms of the load voltage and current in case of slow

demagnetization.

Figure 13. VLOAD and ILOAD in case of slow demagnetization

ILOADx

VLOADX

VINX

ON

ON

OFF

VCC

VCC+VFCD

TDEMAG

time

time

time

If VZ pins are left floating (see fig 14 Active clamp equivalent principle schematic. ) or connected by a Zener or

TVS diode to supply rail (see fig 5 Application circuit (fast decay enabled by TVS between Vz and supply rail) ) or

PGND (see fig 6 Application circuit (fast decay enabled by TVS between VZ and PGND). then the fast decay is

activated. When VZ pins are left floating the integrated clamping circuit protects the IC despite overvoltages: the

conduction state of the integrated switches is modulated in order to keep the LOADx pin voltage < Vdemag until the

energy in the load has been dissipated. The demagnetization energy is dissipated in the IC and it is limited by the

internal heatsink capability, see EAS in Table 2. Absolute maximum ratings .

Figure 14. Active clamp equivalent principle schematic.

LOADX

VCC

PGND

IPU

SUPPLY RAIL

GROUND PLANE

APPLICATION BOARD

IPS4260L

.

.

.

.

.

.

LOAD

RPD

VCC

Active Clamp

Circuitry

VZ

IPS4260L

Active clamp

DS12305 - Rev 5

page 18/29