LM26420YMHX/NOPB National Semiconductor, LM26420YMHX/NOPB Datasheet - Page 12

IC REG DUAL SYNC BUCK 2A 20TSSOP

LM26420YMHX/NOPB

Manufacturer Part Number
LM26420YMHX/NOPB
Description
IC REG DUAL SYNC BUCK 2A 20TSSOP
Manufacturer
National Semiconductor
Series
PowerWise®r
Type
Step-Down (Buck)r
Datasheet

Specifications of LM26420YMHX/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
20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-
Other names
LM26420YMHX
www.national.com
Applications Information
THEORY OF OPERATION
The LM26420 is a constant frequency dual PWM buck syn-
chronous regulator IC that delivers two 2.0A load currents.
The regulator has a preset switching frequency of 2.2MHz or
550kHz. This high frequency allows the LM26420 to operate
with small surface mount capacitors and inductors, resulting
in a DC/DC converter that requires a minimum amount of
board space. The LM26420 is internally compensated, so it
is simple to use and requires few external components. The
LM26420 uses current-mode control to regulate the output
voltage. The following operating description of the LM26420
will refer to the Simplified Block Diagram (Figure 1), which
depicts the functional blocks for one of the two channels, and
to the waveforms in Figure 2. The LM26420 supplies a regu-
lated output voltage by switching the internal PMOS and
NMOS switches at constant frequency and variable duty cy-
cle. A switching cycle begins at the falling edge of the reset
pulse generated by the internal clock. When this pulse goes
low, the output control logic turns on the internal PMOS con-
trol switch (TOP Switch). During this on-time, the SW pin
voltage (V
tor current (I
by the current sense amplifier, which generates an output
proportional to the switch current. The sense signal is
summed with the regulator’s corrective ramp and compared
to the error amplifier’s output, which is proportional to the dif-
ference between the feedback voltage and V
PWM comparator output goes high, the TOP Switch turns off
and the NMOS switch (BOTTOM Switch) turns on after a short
delay, which is controlled by the Dead-Time-Control Logic,
until the next switching cycle begins. During the top switch off-
time, inductor current discharges through the BOTTOM
Switch, which forces the SW pin to swing to ground. The reg-
ulator loop adjusts the duty cycle (D) to maintain a constant
output voltage.
SOFT-START
This function forces V
ing start up in a controlled fashion, which helps reduce inrush
SW
L
) swings up to approximately V
) increases with a linear slope. I
FIGURE 2. Typical Waveforms
OUT
to increase at a controlled rate dur-
IN
, and the induc-
REF
L
is measured
. When the
30069666
12
current and eliminate overshoot on V
the error amplifier’s reference voltage ramps from 0V to its
nominal value of 0.8V in approximately 600 µs. If the con-
verter is turned on into a pre-biased condition then the feed-
back will begin ramping from the pre-bias voltage but at the
same rate as if it had started from 0V. The two outputs startup
ratiometrically if enabled at the same time, see figure below.
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
internal reference, the internal PMOS control switch is turned
off, which allows the output voltage to decrease toward reg-
ulation.
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
sis prevents the part from turning off during power up if V
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 approxi-
mately 150°C.
POWER GOOD
The LM26420 features and 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 high when the output is within the PG tolerance window.
Typical values for this resistor range from 10 kΩ to 100 kΩ.
REF
. Once the FB pin voltage goes 15% above the
IN
drops below 2.3V (typ). Hystere-
OUT
. During soft-start,
PG
) to pull
IN
is

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