LM26420YSQX/NOPB National Semiconductor, LM26420YSQX/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM26420YSQX/NOPB
• Description: IC REG DUAL SYNC BUCK 2A 16LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM26420YSQX/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 4.5 V
• Current - Output: 2A
• Frequency - Switching: 550kHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-LLP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM26420YSQX

The LM26420's power good feature is design with hysterysis
in order to insure no false power good flags are asserted dur-
ing large transient. Once power good is asserted high, it will
not be pulled low until the output voltage exceeds +/-14% of
the setpoint for a during of ~7.5µS (typ.), see figure below.
OVER-CURRENT PROTECTION
When the switch current reaches the current limit value, it im-
mediately is turned off. This effectively reduces the duty cycle
and therefore the output voltage dips and continues to droop
until the output load matches the peak current limit inductor
current. As the FB voltage drops below 480mV the operating
frequency begins to decrease until it hits full on frequency
FIGURE 10. Power Good Hysterysis Operation
FIGURE 9. V
IN
controlling V
OUT
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fold-back which is set to approximately 150kHz for the Y ver-
sion and 300kHz for the X version. Frequency fold back helps
reduce the thermal stress in the IC by reducing the switching
losses and to prevent runaway of the inductor current when
the output is shorted to ground.
It is important to note that when recovering from a over-cur-
rent condition the converter does not go through the soft-start
process. There may be an over shoot due to the sudden re-
moval of the over-current fault. The reference voltage at the
non-inverting input of the error amplifier always sits at 0.8V
during the over-current condition, therefore when the fault is
removed the converter bring the FB voltage back to 0.8V as
quickly as possible. The over-shoot depend on whether there
is a load on the output after the removal of the over-current
fault, the size of the inductor, and the amount of capacitance
on the output. The small the inductor and the larger the ca-
pacitance on the output the small the overshoot. Note, over-
current protection for each output is independent.
PCB LAYOUT CONSIDERATIONS
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most impor-
tant consideration is the close coupling of the GND connec-
tions of the input capacitor and the PGND pin. These ground
ends should be close to one another and be connected to the
GND plane with at least two through-holes. Place these com-
ponents as close to the IC as possible. Next in importance is
the location of the GND connection of the output capacitor,
which should be near the GND connections of VIND and
PGND. There should be a continuous ground plane on the
bottom layer of a two-layer board except under the switching
node island. The FB pin is a high impedance node and care
should be taken to make the FB trace short to avoid noise
pickup and inaccurate regulation. The feedback resistors
should be placed as close as possible to the IC, with the GND
of R1 placed as close as possible to the GND of the IC. The
V
any other traces that are switching. High AC currents flow
through the V
short and wide as possible. However, making the traces wide
increases radiated noise, so the designer must make this
trade-off. Radiated noise can be decreased by choosing a
shielded inductor. The remaining components should also be
placed as close as possible to the IC. Please see Application
Note AN-1229 for further considerations and the LM26420
demo board as an example of a four-layer layout.
OUT
trace to R2 should be routed away from the inductor and
IN
, SW and V
OUT
traces, so they should be as
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