AP6503 Datasheet, PDF (8/13 Pages) Diodes Incorporated – 340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6503
340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
Applications Information (cont.)
Thermal Shutdown
The AP6503 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 150°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 0.925V to 18V
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
=
R
2
⋅ ⎜⎛
⎝
V_OUT
0.925
−1⎟⎞
⎠
Figure 4. Feedback Divider Network
When output voltage is low, network as shown in Figure 4
is recommended.
Vout(V)
5
3.3
2.5
1.8
1.2
R1(KΩ)
45.3
26.1
16.9
9.53
3
R2(KΩ)
10
10
10
10
10
Table 1—Resistor Selection for Common Output
Voltages
Compensation Components
The AP6503 has an external COMP pin through which
system stability and transient response can be controlled.
COMP pin is the output of the internal trans-conductance
error amplifier. A series capacitor-resistor combination
sets a pole-zero combination to control the characteristics
of the control system. The DC gain of the voltage feedback
loop is given by:
A _VDC
=
RLOAD
×
GCS
×
A _VEA
×
VFB
VOUT
Where V_FB is the feedback voltage (0.925V), R_LOAD is the
load resistor value, G_CS is the current sense trans-
conductance and A_VEA is the error amplifier voltage gain.
The control loop transfer function incorporates two poles
one is due to the compensation capacitor (C3) and the
output resistor of error amplifier, and the other is due to
the output capacitor and the load resistor. These poles are
located at:
fP1
=
GEA
2π × C3 × A _VEA
fP2
=
1
2π × C2 × RLOAD
Where G_EA is the error amplifier trans-conductance.
One zero is present due to the compensation capacitor
(C3) and the compensation resistor (R3). This zero is
located at:
fZ1
=
2π
×
1
C3 ×
R3
The goal of compensation design is to shape the converter
transfer function to get a desired loop gain. The system
crossover frequency where the feedback loop has the
unity gain is crucial.
A rule of thumb is to set the crossover frequency to below
one-tenth of the switching frequency. Use the following
procedure to optimize the compensation components:
1. Choose the compensation resistor (R3) to set the
desired crossover frequency. Determine the R3 value by
the following equation:
R3 = ^{2π × C2 × fc} × ^VOUT < ^{2π × C2 × 0.1× fs} × ^VOUT
GEA × GCS VFB
G
EA
×G
CS
VFB
Where f_C is the crossover frequency, which is typically less
than one tenth of the switching frequency.
AP6503
Document number: DS35077 Rev. 1 - 2
9 of 13
www.diodes.com
September 2011
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