Datasheet

VOLTAGE
TIME
RATIOMETRIC START UP
V
OUT1
V
OUT2
V
EN1,2
LM26420
SNVS579F –FEBRUARY 2009–REVISED MARCH 2013 www.ti.com
Figure 34.
OUTPUT OVER-VOLTAGE PROTECTION
The over-voltage comparator compares the FB pin voltage to a voltage that is approximately 15% higher than the
internal reference V
REF
. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS
control switch is turned off, which allows the output voltage to decrease toward regulation.
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout (UVLO) prevents the LM26420 from operating until the input voltage exceeds 2.628V (typ).
The UVLO threshold has approximately 330 mV of hysteresis, so the part will operate until V
IN
drops below 2.3V
(typ). Hysteresis prevents the part from turning off during power up if V
IN
is non-monotonic.
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 3.3A (typ), and turns off the switch until the
next switching cycle begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off the output switch when the IC 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.
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 (R
PG
) 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.
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 1.04V and has a 150mV hysteresis.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
OUT
) to input voltage (V
IN
):
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