Diodes AP6508 - Manual (Page 9)

AP6508
500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
AP6508
Document number: DS33437 Rev. 5 - 2
9 of 14
www.diodes.com
October 2011
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Application Information
Theory of Operation
The AP6508 is a 3A current mode, synchronous buck
regulator with built in power MOSFETs. current mode
control assures excellent line and load regulation and a
wide loop bandwidth for fast response to load transients.
Figure. 2 depicts the functional block diagram of AP6508.
The operation of one switching cycle can be explained as
follows. At the beginning of each cycle, HS (high-side)
MOSFET is off. The EA output voltage is higher than the
current sensing amplifier output, and the current
comparator’s output is low. The rising edge of the 500kHz
oscillator clock signal sets the RS Flip-Flop. Its output
turns on HS MOSFET. The current sensing amplifier is
reset for every switching cycle.
When the HS MOSFET is on, inductor current starts to
increase. The current sensing amplifier senses and
amplifies the inductor current. Since the current mode
control is subject to sub-harmonic oscillations that peak at
half the switching frequency, slope compensation is
utilized. This will help to stabilize the power supply. This
slope compensation is summed to the current sensing
amplifier output and compared to the error amplifier output
by the PWM comparator. When the sum of the current
sensing amplifier output and the slope compensation
signal exceeds the EA output voltage, the RS Flip-Flop is
reset and HS MOSFET is turned off.
For one whole cycle, if the sum of the current sensing
amplifier output and the slope compensation signal does
not exceed the EA output, then the falling edge of the
oscillator clock resets the flip-flop. The output of the error
amplifier increases when feedback voltage (VFB) is lower
than the reference voltage of 0.807V. This also increases
the inductor current as it is proportional to the EA voltage.
When the HS MOSFET turns off, the synchronous LS
MOSFET turns on until the next clock cycle begins. There
is a “dead time” between the HS turn off and LS turn on
that prevents the switches from “shooting through” from
the input supply to ground.
The voltage loop is internally compensated with the 50pF
and 200k
Ω RC network. The maximum EAMP voltage
output is precisely clamped at 2.1V.
Internal Regulator
Most of the internal circuitry including the bottom driver
are powered from the 5V internal regulator. When Vin is
less than 5V, this internal regulator cannot maintain the
5V regulation and hence the output voltage would also
drop from regulation.
Enable
The enable (EN) input allows the user to control turning
on or off the converter. To enable the converter EN must
be pulled above the ‘EN Rising Threshold’ and to disable
the converter EN must be pulled below ‘EN falling
Threshold’ (EN rising threshold – EN threshold
Hysteresis).
Few conditions on EN function:
1) EN must be pulled low for at least 5us to disable the
regulator.
2) The voltage on EN cannot exceed 5V.
3) The AP6508 can be enabled by Vin through a voltage
divider as shown in the figure 1 below.
Figure 1. EN Divider Network
1M Ω || R
1M Ω || R (R
V V
BOT
BOT TOP
RISE EN RISE IN
+
=
− −
Where
RISE EN
V − = 1.3V(TYP)
1M Ω || R
1M Ω || R (R
V V
BOT
BOT TOP
FALL EN FALL IN
+
=
− −
Where
FALL EN
V − = 0.9V(TYP)
Power Good
Power Good (PGOOD) is an open drain and active high
output. This output can be pulled up high to the
appropriate level with an external resistor. The PGOOD
is flagged low when Vfb=0.7V and is an open drain
output when Vfb=0.9V. The PGOOD output can deliver
a max of 4 mA sink current at 0.4 V when de- asserted.
The PGOOD pin is held low during soft-start. Once
output voltage reaches 90% of its final value, PGOOD
goes high if there are no faults.