Diodes AP6507 - Manual (Page 10)

AP6507
500 kHz 18V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
AP6507
Document number: DS33435 Rev. 3 - 2
10 of 13
www.diodes.com
October 2011
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Application Information (cont.)
Current Limit Protection
In order to reduce the total power dissipation and to
protect the application, AP6507 has cycle-by-cycle current
limiting implementation. The voltage drop across the
internal high-side 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
AP6507 has a UVLO comparator that monitors the input
voltage and the internal bandgap reference. If the input
voltage falls below 4.0V, the AP6507 will latch an under
voltage fault. In this event the AP6507 will be disabled and
power has to be re-cycled to reset the UVLO fault.
Thermal Shutdown
The AP6507 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 0.808V 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
0.808
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)
+ =