Diodes AP6508 - Manual (Page 10)

AP6508
500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
AP6508
Document number: DS33437 Rev. 5 - 2
10 of 14
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October 2011
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Application Information (cont.)
External Soft Start
Soft start is traditionally implemented to prevent the
excess inrush current. This in turn prevents the converter
output voltage from overshooting when it reaches
regulation. The AP6508 has an internal current source
with a soft start capacitor to ramp the reference voltage
from 0V to 0.807V. The soft start time is internally fixed at
2ms (TYP). The soft start time can be extended > 2ms by
adding a soft start capacitor externally. The soft start
sequence is reset when there is a thermal shutdown,
Under Voltage Lockout (UVLO) or when the part is
disabled using the EN pin.
External soft start can be calculated from the formula
below:
DT
DV
* C I
SS
=
Where;
Iss = Soft Start Current
C = External Capacitor
DV=change in feedback voltage from 0V to maximum
voltage
DT = Soft Start Time
Current Limit Protection
The AP6508 has cycle-by-cycle current limiting
implementation. The voltage drop across the internal HS
MOSFET is sensed and compared with the internally set
current limit threshold. This voltage drop is sensed at
about 30ns after the HS turns on. This voltage drop is
proportional to the peak inductor current. When the peak
inductor current exceeds the set current limit threshold,
current limit protection is activated. During this time the
feedback voltage (VFB) drops down. When the voltage at
the FB pin reaches 0.3V, the internal oscillator shifts the
frequency from the normal operating frequency of 500kHz
to a fold-back frequency of 150kHz. The current limit is
reduced to 70% of nominal current limit when the part is
operating at 150kHz. This low fold-back frequency
prevents current runaway.
Under Voltage Lockout (UVLO)
Under Voltage Lockout is implemented to prevent the
IC from operating under insufficient input voltages. The
AP6508 has a UVLO comparator that monitors the input
voltage and internal bandgap reference. If the input
voltage falls below 3.8V, the AP6508 will latch an under
voltage fault. In this event the AP6508 will be disabled
and power has to be re-cycled to reset the UVLO fault.
Thermal Shutdown
The AP6508 has on-chip thermal protection that prevents
damage to the IC when the die temperature exceeds safe
margins. It implements a thermal sensing to monitor the
operating junction temperature of the IC. Once the die
temperature rises to approximately 140°C, the thermal
protection feature gets activated .The internal thermal
sense circuitry turns the IC off thus preventing the power
switch from damage.
A hysteresis in the thermal sense circuit allows the
device to cool down to approximately 120°C before the
IC is enabled again through soft start. This thermal
hysteresis feature prevents undesirable oscillations of
the thermal protection circuit.
Setting the Output Voltage
The output voltage can be adjusted from 0.807V to 15V
using an external resistor divider. Table 1 shows a list of
resistor selection for common output voltages. Resistor
R1 is selected based on a design tradeoff between
efficiency and output voltage accuracy. For high values
of R1 there is less current consumption in the feedback
network. However the trade off is output voltage
accuracy due to the bias current in the error amplifier. R2
can be determined by the following equation:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
− ⋅ =
1
0.807
V
R R
OUT
2 1
Figure 2. Feedback Divider Network
When output voltage is low, a T-type network as shown
in Figure 2 is recommended.
V
OUT
(V) R
1
(k Ω) R
2
(k Ω) R
t
(k Ω)
1.2 4.99 10.2 24.9
1.8 4.99
(1%) 4.02 (1%) 35.7
2.5 40.2 (1%) 19.1 (1%) 24.9
3.3 40.2 (1%) 13 (1%) 24.9
5 40.2 (1%) 7.68 (1%) 35.7
Table 1.Resistor Selection for Common Output
Voltages
Inductor
Calculating the inductor value is a critical factor in
designing a buck converter. For most designs, the
following equation can be used to calculate the inductor
value;
SW L IN
OUT IN OUT
f ΔI V
) V (V V
L
⋅ ⋅
− ⋅
=
Where
L
ΔI
is the inductor ripple current.
And
SW
f is the buck converter switching frequency.
Choose the inductor ripple current to be 30% of the
maximum load current. The maximum inductor peak
current is calculated from:
2
ΔI
I I L
LOAD L(MAX)
+ =