LM2651-3.3EVAL National Semiconductor, LM2651-3.3EVAL Datasheet - Page 8

• Manufacturer Part Number: LM2651-3.3EVAL
• Description: EVALUATION BOARD FOR LM2651-3.3
• Manufacturer: National Semiconductor

Specifications of LM2651-3.3EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 1.5A
• Voltage - Input: 4 ~ 14V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2651
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2651-3.3EVAL

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Block Diagram
Operation
The LM2651 operates in a constant frequency (300 kHz),
current-mode PWM for moderate to heavy loads; and it
automatically switches to hysteretic mode for light loads. In
hysteretic mode, the switching frequency is reduced to keep
the efficiency high.
MAIN OPERATION
When the load current is higher than the sleep mode thresh-
old, the part is always operating in PWM mode. At the
beginning of each switching cycle, the high-side switch is
turned on, the current from the high-side switch is sensed
and compared with the output of the error amplifier (COMP
pin). When the sensed current reaches the COMP pin volt-
age level, the high-side switch is turned off; after 40 ns
(deadtime), the low-side switch is turned on. At the end of the
switching cycle, the low-side switch is turned off; and the
same cycle repeats.
The current of the top switch is sensed by a patented internal
circuitry. This unique technique gets rid of the external sense
Design Procedure
This section presents guidelines for selecting external com-
ponents.
FIGURE 1. LM2651 Block Diagram
10092503
8
resistor, saves cost and size, and improves noise immunity
of the sensed current. A feedforward from the input voltage is
added to reduce the variation of the current limit over the
input voltage range.
When the load current decreases below the sleep mode
threshold, the output voltage will rise slightly, this rise is
sensed by the hysteretic mode comparator which makes the
part go into the hysteretic mode with both the high and low
side switches off. The output voltage starts to drop until it hits
the low threshold of the hysteretic comparator, and the part
immediately goes back to the PWM operation. The output
voltage keeps increasing until it reaches the top hysteretic
threshold, then both the high and low side switches turn off
again, and the same cycle repeats.
PROTECTIONS
The cycle-by-cycle current limit circuitry turns off the high-
side MOSFET whenever the current in MOSFET reaches
2A.