LM2614ATLX NSC [National Semiconductor], LM2614ATLX Datasheet - Page 10

• Manufacturer Part Number: LM2614ATLX
• Description: 400mA Sub-Miniature Adjustable DC-DC Converter Optimized for RF Power Amplifiers
• Manufacturer: NSC [National Semiconductor]

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Device Information
PWM OPERATION
While in PWM (Pulse Width Modulation) mode, the output
voltage is regulated by switching at a constant frequency
and then modulating the energy per cycle to control power to
the load. Energy per cycle is set by modulating the PFET
switch on-time pulse-width to control the peak inductor cur-
rent. This is done by comparing the signal from the
current-sense amplifier with a slope compensated error sig-
nal from the voltage-feedback error amplifier. At the begin-
ning of each cycle, the clock turns on the PFET switch,
causing the inductor current to ramp up. When the current
sense signal ramps past the error amplifier signal, the PWM
comparator turns off the PFET switch and turns on the NFET
synchronous rectifier, ending the first part of the cycle. If an
increase in load pulls the output voltage down, the error
amplifier output increases, which allows the inductor current
to ramp higher before the comparator turns off the PFET.
This increases the average current sent to the output and
adjusts for the increase in the load.
Before going to the PWM comparator, the error signal is
summed with a slope compensation ramp from the oscillator
for stability of the current feedback loop. During the second
part of the cycle, a zero crossing detector turns off the NFET
synchronous rectifier if the inductor current ramps to zero.
The minimum on time of the PFET in PWM mode is about
200ns.
A: Inductor Current, 500mA/div
B: SW Pin, 2V/div
C: V
OUT
, 10mV/div, AC Coupled
PWM Mode Switching Waveform
FIGURE 6.
(Continued)
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10
A: Inductor Current, 500mA/div
B: SW Pin, 2V/div
C: V
PFM OPERATION
Connecting the SYNC/MODE to SGND sets the LM2614 to
hysteretic PFM operation. While in PFM (Pulse Frequency
Modulation) mode, the output voltage is regulated by switch-
ing with a discrete energy per cycle and then modulating the
cycle rate, or frequency, to control power to the load. This is
done by using an error comparator to sense the output
voltage. The device waits as the load discharges the output
filter capacitor, until the output voltage drops below the lower
threshold of the PFM error-comparator. Then the device
initiates a cycle by turning on the PFET switch. This allows
current to flow from the input, through the inductor to the
output, charging the output filter capacitor. The PFET is
turned off when the output voltage rises above the regulation
threshold of the PFM error comparator. Thus, the output
voltage ripple in PFM mode is proportional to the hysteresis
of the error comparator.
In PFM mode, the device only switches as needed to service
the load. This lowers current consumption by reducing power
consumed during the switching action in the circuit, due to
transition losses in the internal MOSFETs, gate drive cur-
rents, eddy current losses in the inductor, etc. It also im-
proves light-load voltage regulation. During the second half
of the cycle, the intrinsic body diode of the NFET synchro-
nous rectifier conducts until the inductor current ramps to
zero.
OPERATING MODE SELECTION
The LM2614 is designed for digital control of the operating
modes by the system controller. This prevents the spurious
switch over from low-noise PWM mode between transmis-
sion intervals in mobile phone applications that can occur in
other products.
The SYNC/MODE digital input pin is used to select the
operating mode. Setting SYNC/MODE high (above 1.3V)
selects 600kHz current-mode PWM operation. PWM mode
is optimized for low-noise, high-power operation for use
when the load is active. Setting SYNC/MODE low (below
0.4V) selects hysteretic voltage-mode PFM operation. PFM
mode is optimized for reducing power consumption and
OUT
, 50mV/div, AC Coupled
PFM Mode Switching Waveform
FIGURE 7.
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