AAT2514_08 Datasheet, PDF (8/14 Pages) Advanced Analogic Technologies – Dual Channel 600mA Step-Down Converter

SwitchReg^TM
PRODUCT DATASHEET
AAT2514
Dual Channel 600mA Step-Down Converter
Current Mode PWM Control
Slope compensated current mode PWM control provides
stable switching and cycle-by-cycle current limit for
excellent load and line response and protection of the
internal main switch (P-channel MOSFET) and synchro-
nous rectifier (N-channel MOSFET). During normal opera-
tion, the internal P-channel MOSFET is turned on for a
specified time to ramp the inductor current at each rising
edge of the internal oscillator, and is switched off when
the peak inductor current is above the error voltage. The
current comparator, I_COMP, limits the peak inductor cur-
rent. When the main switch is off, the synchronous recti-
fier turns on immediately and stays on until either the
inductor current starts to reverse, as indicated by the
current reversal comparator, I_ZERO, or the beginning of the
next clock cycle. The OVDET comparator controls output
transient overshoot by turning the main switch off and
keeping it off until the fault is no longer present.
Control Loop
The AAT2514 is a peak current mode step-down converter.
The current through the P-channel MOSFET (high side) is
sensed for current loop control, as well as short circuit and
overload protection. An adaptive slope compensation sig-
nal is added to the sensed current to maintain stability for
duty cycles greater than 50%. The peak current mode
loop appears as a voltage-programmed current source in
parallel with the output capacitor. The output of the volt-
age error amplifier programs the current mode loop for
the necessary peak switch current to force a constant
output voltage for all load and line conditions. Internal
loop compensation terminates the transconductance volt-
age error amplifier output. For fixed voltage versions, the
error amplifier reference voltage is internally set to pro-
gram the converter output voltage. For the adjustable
output, the error amplifier reference is fixed at 0.6V.
Enable
The enable pins are active high. When pulled low, the
enable input forces the AAT2514 into a low-power, non-
switching state. The total input current during shutdown
is less than 2μA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles. Thermal protec-
tion completely disables switching when internal dissipa-
tion becomes excessive. The junction over-temperature
threshold is 140°C with 15°C of hysteresis. Once an
over-temperature or over-current fault conditions is
removed, the output voltage automatically recovers.
Dropout Operation
When the input voltage decreases toward the value of
the output voltage, the AAT2514 allows the main switch
to remain on for more than one switching cycle and
increases the duty cycle until it reaches 100%.
The duty cycle D of a step-down converter is defined as:
D = TON · F_OSC · 100% ≈
VOUT _{· 100%}
V_IN
Where T_ON is the main switch on time and F_OSC is the
oscillator frequency (1.5MHz).
The output voltage then is the input voltage minus the
voltage drop across the main switch and the inductor. At
low input supply voltage, the R_DS(ON) of the P-channel
MOSFET increases and the efficiency of the converter
decreases. Caution must be exercised to ensure the heat
dissipated does not exceed the maximum junction tem-
perature of the IC.
Maximum Load Current
The AAT2514 will operate with an input supply voltage as
low as 2.5V; however, the maximum load current
decreases at lower input due to the large IR drop on the
main switch and synchronous rectifier. The slope com-
pensation signal reduces the peak inductor current as a
function of the duty cycle to prevent sub-harmonic oscil-
lations at duty cycles greater than 50%. Conversely, the
current limit increases as the duty cycle decreases.
8
www.analogictech.com
2514.2008.02.1.1