AME5252 Datasheet, PDF (10/18 Pages) Analog Microelectronics – Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Converter

AME
AME5252
Dual Synchronous, 600mA, 1.5MHz
Step-Down DC/DC Converter
n Detailed Description
Dropout Operation
The AME5252 uses a constant frequency, current mode
architecture. The operating frequency is set at 1.5MHz
and can be synchronized to an external oscillator. Both
channels share the same clock and run in-phase.
The output voltage is set by an external divider returned
to the V_FB pins. An error amplifier compares the divided
output voltage with a reference voltage of 0.6V and adjusts
the peak inductor current accordingly. Overvoltage and
undervoltage comparators will pull the PORB output low if
the output voltage is not within 8.5%. The PORB output
will go high after 262,144 clock cycles (about 175ms) of
achieving regulation.
Main Control Loop
During normal operation, the top power switch (P-chan-
nel MOSFET) is turned on at the beginning of a clock
cycle when the V_FB voltage is below the reference voltage.
The current into the inductor and the load increases until
the current limit is reached. The switch turns off and en-
ergy stored in the inductor flows through the bottom switch
(N-channel MOSFET) into the load until the next clock
cycle.
The peak inductor current is controlled by the internally
compensated COMP voltage, which is the output of the
error amplifier. This amplifier compares the V_FB pin to the
0.6V reference. When the load current increases, the V_FB
voltage decreases slightly below the reference. This de-
crease causes the error amplifier to increase the COMP
voltage until the average inductor current matches the new
load current. The main control loop is shut down by pull-
ing the EN pin to ground.
Short-Circuit Protection
When the output is shorted to ground, the frequency of
the oscillator is reduced to about 210kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the in-
ductor current has more time to decay, thereby preventing
runaway. The oscillator's frequency will progressively in-
crease to 1.5MHz when V_FB or V_OUT rises above 0V.
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which is
the dropout condition. In dropout, the P-channel MOSFET
switch is turned on continuously with the output voltage
being equal to the input voltage minus the voltage drops
across the internal P-channel MOSFET and the inductor.
An important design consideration is that the R_DSON of
the P-channel switch increases with decreasing input sup-
ply voltage (See Typical Performance Characteristics).
Therefore, the user should calculate the power dissipa-
tion when the AME5252 is used at 100% duty cycle with
low input voltage.
n Application Information
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 1µH to 4.7µH. Its value is chosen based on
the desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher V_IN or V_OUT also increases the ripple cur-
rent as shown in equation 1. A reasonable starting point
for setting ripple current is IL = 240mA (40% of 600mA).
∆ IL=
1
f⋅
L
⋅
V_OUT
(1 − ^VOUT
V_IN
)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
i1n2d0umctAo)r.sFhoorubldetbteereenffoicuiegnhc_cfyo,r
most applications (600mA+
choose a low DC-resistance
inductor.
10
Rev. C.01