AP6508 Datasheet, PDF (10/14 Pages) Diodes Incorporated – 500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6508
500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
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:
ISS
=
C
*
DV
DT
Where;
Iss = Soft Start Current
C = External Capacitor
DV=change in output voltage from 0V to maximum voltage
DT = Soft Start Time
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 0.81V 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:
R1
=
R2
⋅
⎜⎜⎝⎛
VOUT
0.81
− 1_{⎟⎟⎠⎞}
Current Limit Protection
The AP6508 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. 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 runaway
current.
Under Voltage Lockout (UVLO)
Under Voltage Lockout is implemented to prevent the
IC from insufficient input voltages. The AP6508 has a
UVLO comparator that monitors the inout 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 output will be pulled low 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.
Figure 2. Feedback Divider Network
When output voltage is low, T-type network as shown in
Figure 2 is recommended.
V_OUT (V)
1.2
1.8
2.5
3.3
5
R₁ (kΩ)
4.99
4.99 (1%)
40.2 (1%)
40.2 (1%)
40.2 (1%)
R₂ (kΩ)
10.2
4.02 (1%)
19.1 (1%)
13 (1%)
7.68 (1%)
R_t (kΩ)
24.9
24.9
0
10
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;
L = ^{VOUT ⋅ (VIN − VOUT )}
V_IN ⋅ ΔI_L ⋅ f_SW
Where ΔI_L is the inductor ripple current.
And f_SW 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:
I_L(MAX)
= I_LOAD
+
ΔI_L
2
AP6508
Document number: DS33437 Rev. 4 - 2
10 of 14
www.diodes.com
July 2011
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