LM26420, LM26420-Q0, LM26420-Q1
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SNVS579J –FEBRUARY 2009–REVISED SEPTEMBER 2015
Feature Description (continued)
RATIOMETRIC START UP
V
OUT1
V
OUT2
V
EN1,2
TIME
Figure 35. LM26420 Soft-Start
7.3.2 Power Good
The LM26420 features an open drain power good (PG) pin to sequence external supplies or loads and to provide
fault detection. This pin requires an external resistor (RPG) to pull PG high when the output is within the PG
tolerance window. Typical values for this resistor range from 10 kΩ to 100 kΩ.
7.3.3 Precision Enable
The LM26420 features independent precision enables that allow the converter to be controlled by an external
signal. This feature allows the device to be sequenced either by a external control signal or the output of another
converter in conjunction with a resistor divider network. It can also be set to turn on at a specific input voltage
when used in conjunction with a resistor divider network connected to the input voltage. The device is enabled
when the EN pin exceeds 1.04 V and has a 150-mV hysteresis.
7.4 Device Functional Modes
7.4.1 Output Overvoltage Protection
The overvoltage comparator compares the FB pin voltage to a voltage that is approximately 15% greater than the
internal reference VREF. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS
switch is turned off, which allows the output voltage to decrease toward regulation.
7.4.2 Undervoltage Lockout
Undervoltage lockout (UVLO) prevents the LM26420 from operating until the input voltage exceeds 2.628 V
(typical). The UVLO threshold has approximately 330 mV of hysteresis, so the part will operate until VIN drops
below 2.3 V (typical). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic.
7.4.3 Current Limit
The LM26420 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a
current limit comparator detects if the output switch current exceeds 3.3 A (typical), and turns off the switch until
the next switching cycle begins.
7.4.4 Thermal Shutdown
Thermal shutdown limits total power dissipation by turning off the output switch when the device junction
temperature exceeds 165°C. After thermal shutdown occurs, the output switch does not turn on until the junction
temperature drops to approximately 150°C.
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