LM5109B
SNVS477C –FEBRUARY 2007–REVISED JANUARY 2016
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Table 2. VHB-HS UVLO Feature Logic Operation
CONDITION (VDD > VDDR) HI LI HO LO
VHB-HS < VHBR during device start-up H L L L
VHB-HS < VHBR during device start-up L H L H
VHB-HS < VHBR during device start-up H H L H
VHB-HS < VHBR during device start-up L L L L
VHB-HS < VHBR – VHBH after device start-up H L L L
VHB-HS < VHBR – VHBH after device start-up L H L H
VHB-HS < VHBR – VHBH after device start-up H H L H
VHB-HS < VHBR – VHBH after device start-up L L L L
7.3.2 Level Shift
The level shift circuit is the interface from the high-side input to the high-side driver stage which is referenced to
the switch node (HS). The level shift allows control of the HO output which is referenced to the HS pin and
provides excellent delay matching with the low-side driver.
7.3.3 Output Stages
The output stages are the interface to the power MOSFETs in the power train. High slew rate, low resistance,
and high-peak current capability of both outputs allow for efficient switching of the power MOSFETs. The low-
side output stage is referenced to VSS and the high-side is referenced to HS.
7.4 Device Functional Modes
The device operates in normal mode and UVLO mode. See Start-Up and UVLO for more information on UVLO
operation mode. In normal mode when the VDD and VHB–HS are above UVLO threshold, the output stage is
dependent on the states of the HI and LI pins. The output HO and LO will be low if input state is floating.
Table 3. INPUT and OUTPUT Logic Table
HI LI HO(1) LO(2)
L L L L
L H L H
H L H L
H H H H
Floating Floating L L
(1) HO is measured with respect to the HS.
(2) LO is measured with respect to the VSS.
7.5 HS Transient Voltages Below Ground
The HS node will always be clamped by the body diode of the lower external FET. In some situations, board
resistances and inductances can cause the HS node to transiently swing several volts below ground. The HS
node can swing below ground provided:
1. HS must always be at a lower potential than HO. Pulling HO more than –0.3 V below HS can activate
parasitic transistors resulting in excessive current flow from the HB supply, possibly resulting in damage to
the IC. The same relationship is true with LO and VSS. If necessary, a Schottky diode can be placed
externally between HO and HS or LO and GND to protect the IC from this type of transient. The diode must
be placed as close to the IC pins as possible to be effective.
2. HB to HS operating voltage must be 15 V or less. Hence, if the HS pin transient voltage is –5 V, VDD must
be ideally limited to 10 V to keep HB to HS below 15 V.
3. Low-ESR bypass capacitors from HB to HS and from VDD to VSS are essential for proper operation. The
capacitor must be located at the leads of the IC to minimize series inductance. The peak currents from LO
and HO can be quite large. Any series inductances with the bypass capacitor will cause voltage ringing at the
leads of the IC which must be avoided for reliable operation.
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