LM2642MTC/NOPB National Semiconductor, LM2642MTC/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM2642MTC/NOPB
• Description: IC CTRLR SW SYNC STPDN 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LM2642MTC/NOPB

• Applications: Embedded systems, Console/Set-Top boxes
• Current - Supply: 1mA
• Voltage - Supply: 4.5 V ~ 30 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Dc To Dc Converter Type: Synchronous Step Down Controller
• Number Of Outputs: 2
• Pin Count: 28
• Input Voltage: 4.5 to 30V
• Output Voltage: 1.3 to 30V
• Output Current: 20A
• Package Type: TSSOP
• Mounting: Surface Mount
• Operating Temperature Classification: Automotive
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Primary Input Voltage: 30V
• No. Of Outputs: 1
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 3 - 168 Hours
• Control Mode: Current
• Rohs Compliant: Yes
• For Use With: LM2642REVD EVAL - BOARD EVALUATION LM2642
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM2642MTC; *LM2642MTC/NOPB; LM2642MTC

Operation Descriptions
SOFT START
The ON/SS1 pin has dual functionality as both channel
enable and soft start control. The soft start block diagram is
shown in Figure 3.
The LM2642 will remain in shutdown mode while both soft
start pins are grounded.In a normal application (with a soft
start capacitor connected between the ON/SS1 pin and
SGND) soft start functions as follows. As the input voltage
rises (note: Iss starts to flow when VIN ≥ 2.2V), the internal
5V LDO starts up, and an internal 2µA current charges the
soft start capacitor. During soft start phase, the error ampli-
fier output voltage at the COMPx pin is clamped at 0.55V
and the duty cycle is controlled only by the soft start voltage.
As the SSx pin voltage ramps up, the duty cycle increases
proportional to the soft start ramp, causing the output voltage
to ramp up. The rate at which the duty cycle increases
depends on the capacitance of the soft start capacitor. The
higher the capacitance, the slower the output voltage ramps
up. When the corresponding output voltage exceeds 98%
(typical) of the set target voltage, the regulator switches from
soft start to normal operating mode. At this time, the 0.55V
clamp at the output of the error amplifier releases and peak
current feedback control takes over. Once in peak current
feedback control mode, the output of the error amplifier will
travel within the 0.5V and 2V window to achieve PWM
control. See Figure 4.
During soft start, over-voltage protection and current limit
remain in effect. The under voltage protection feature is
activated when the ON/SS pin exceeds the timeout thresh-
old (3.3V typical). If the ON/SSx capacitor is too small, the
duty cycle may increase too rapidly, causing the device to
latch off due to output voltage overshoot above the OVP
threshold. This becomes more likely in applications requiring
low output voltage, high input voltage and light load. A
capacitance of 10nF is recommended at each soft start pin
to provide a smooth monotonic output ramp.
FIGURE 3. Soft Start and ON/OFF
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SEQUENTIAL STARTUP
Sequential startup can be implemented by simply connecting
PGOOD1 to SS/ON2. Once channel 1 has reached 94% of
nominal, PGOOD1 will go high, thus enabling SS/ON2. In
this mode of operation, channel 2 will be controlled by the
state of channel 1. If channel 1 falls out of the PGOOD1
window, channel 2 will be switched off immediately.
OVER VOLTAGE PROTECTION (OVP)
If the output voltage on either channel rises above 113% of
nominal, over voltage protection activates. Both channels
will latch off, and the PGOOD1 pin will go low. When the
OVP latch is set, the high side FET driver, HDRVx, is imme-
diately turned off and the low side FET driver, LDRVx, is
turned on to discharge the output capacitor through the
inductor. To reset the OVP latch, either the input voltage
must be cycled, or both channels must be switched off.
UNDER VOLTAGE PROTECTION (UVP) AND UV DELAY
If the output voltage on either channel falls below 80% of
nominal, under voltage protection activates. As shown in
Figure 5, an under-voltage event will shut off the UV_DELAY
MOSFET, which will allow the UV_DELAY capacitor to
charge at 5uA (typical). At the UV_DELAY threshold (2.3V
FIGURE 4. Voltage Clamp at COMPx Pin
FIGURE 5. PGOOD, OVP and UVP
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