LM20146MHEVAL National Semiconductor, LM20146MHEVAL Datasheet - Page 11

• Manufacturer Part Number: LM20146MHEVAL
• Description: BOARD EVALUATION FOR LM20146MH
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20146MHEVAL

• Design Resources: LM20146 Design Spreadsheet
• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 6A
• Voltage - Input: 2.95 ~ 5.5V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: LM20146
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

UVLO
The LM20146 has a built-in under-voltage lockout protection
circuit that keeps the device from switching until the input
voltage reaches 2.7V (typical). The UVLO threshold has 45
mV of hysteresis that keeps the device from responding to
power-on glitches during start up. If desired the turn-on point
of the supply can be changed by using the precision enable
pin and a resistor divider network connected to V
in Figure 7. in the design guide.
THERMAL PROTECTION
Internal thermal shutdown circuitry is provided to protect the
integrated circuit in the event that the maximum junction tem-
perature is exceeded. When activated, typically at 160°C, the
LM20146 tri-states the power FETs and resets soft start. After
the junction cools to approximately 150°C, the part starts up
using the normal start up routine. This feature is provided to
prevent catastrophic failures from accidental device over-
heating.
LIGHT LOAD OPERATION
The LM20146 offers increased efficiency when operating at
light loads. Whenever the load current is reduced to a point
where the peak to peak inductor ripple current is greater than
FIGURE 1. Powergood Behavior
IN
as shown
11
two times the load current, the part will enter the diode emu-
lation mode preventing significant negative inductor current.
The point at which this occurs is the critical conduction bound-
ary and can be calculated by the following equation:
Several diagrams are shown in Figure 2 illustrating continu-
ous conduction mode (CCM), discontinuous conduction
mode, and the boundary condition.
It can be seen that in diode emulation mode, whenever the
inductor current reaches zero the SW node will become high
impedance. Ringing will occur on this pin as a result of the LC
tank circuit formed by the inductor and the parasitic capaci-
tance at the node. If this ringing is of concern an additional
RC snubber circuit can be added from the switch node to
ground.
At very light loads, usually below 100 mA, several pulses may
be skipped in between switching cycles, effectively reducing
the switching frequency and further improving light-load effi-
ciency.
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