AN1084

Application Note 1084
Design Consideration with AP3502F/3F
Prepared by Cheng Zhi Peng
System Engineering Dept.
1. Introduction
The AP3502F/3F are synchronous step-down
converters with internal power MOSFETs. Turn
on/off M1 and M2 alternately to chop the input
voltage. The current sense signal is compared with
the EA output signal to regulate the output voltage
and adjust the MOSFETs’ duty cycle. The
AP3502F/3F are also high reliability ICs with
integrated OCP, OVP, OTP, UVLO circuit. For more
information please refer to the functional block
diagram (Figure 1).
The AP3502F/3F are current-mode step-down
DC-DC converters, capable of driving a 2A/3A load
with high efficiency, excellent line and load
regulation. The AP3502F/3F integrate cycle-by-cycle
current limit protection, programmable soft-start,
short circuit protection and over temperature
protection, which can notably increase the system
reliability.
2. General Description
Figure 1. Functional Block Diagram of AP3502F/3F
Nov. 2012
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BCD Semiconductor Manufacturing Limited
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Application Note 1084
forcing M1 to turn off, and this will last until the next
switching cycle.
2.1 Programmable Soft-start
The soft-start time of the AP3502F/3F is fully user
programmable by selecting different CSS value. The
CSS is charged by a 6μA current source, generating a
ramp signal fed into non-inverting input of the error
amplifier. And this ramp signal will regulate the
voltage on COMP pin when starting the system, thus
realizing soft-start. The capacitor value required for a
given soft-start ramp time can be expressed as:
C SS = t SS ×
2.3 Short Circuit Protection
The VFB is proportional to VOUT. When the output
terminal is shorted and VFB is below 0.3V, the
operating frequency will reduce to 90kHz for system
protection. The AP3502F/3F will restart once
released from OCP condition.
6 μA
VFB
2.4 Over Voltage Protection
The AP3502F/3F have internal OVP circuits. When
VOUT is higher than the OVP threshold, the power
switching will be turned off. The AP3502F/3F will
restart once released from OCP condition.
Where CSS is the required capacitor between SS pin
and GND, tSS is the desired soft-start time and VFB is
the feedback voltage.
2.5 Over Temperature Protection
The OTP circuitry is provided to protect the IC if the
maximum junction temperature is exceeded. When
the junction temperature exceeds 160ºC, it will shut
down the internal control circuit, M1 and M2. The
AP3502F/3F will restart automatically under the
control of soft-start circuit when the junction
temperature decreases to 130ºC/140ºC.
2.2 Over Current Protection
The AP3502F/3F have internal over current
protection function to protect themselves from
catastrophic failure. The AP3502F/3F can monitor
the drain-to-source current of M1. The peak
current-limit threshold is internally set at 3.5/5.6A.
When the inductor current is higher than the current
limit threshold, OCP function will be triggered,
C5 10nF
VIN=12V
R4
100k
7
C1
10 F/25V
(10 F/25V x2)
2
IN
EN
1
BS
AP3502F/3F
8 SS
GND
4
C4
0.1μF
SW
FB 5
COMP
6
C3
4.7nF
(3.3nF)
C6
Optional
L1 10μH
3
VOUT=3.3V
R1 26.1k
D1
Optional
R2
10k
C2
22μF/6.3V x2
R3
13k
C1, C3
A
(B)
A for AP3502F
B for AP3503F
Figure 2. Typical Application of AP3502F/3F
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Rev. 1. 1
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Application Note 1084
3. Application Information
I PEAK = I OUT +
Typical application circuit is shown in the Figure 2,
and for the circuit parameters setting please refer to
the following descriptions.
(VIN − VOUT )× VOUT
2 × VIN × f SW × L1
Where IPEAK is the peak inductor current.
3.1 Output Voltage Setting
The output voltage can be set using a voltage divider
from the output to FB pin. VOUT is divided by the
voltage divider as below:
The current rating of the selected inductor should be
ensured to be 1.5 times of the peak inductor current.
3.3 Input Capacitor Setting
A high-quality input capacitor with big value is
needed to filter noise at input voltage source and limit
the input ripple voltage while supplying most of the
switch current during ON time. For input capacitor
selection, a ceramic capacitor is highly recommended
due to its low impedance and small size. However,
tantalum or low electrolytic capacitor is also sufficed.
⎛ R2 ⎞
V FB = VOUT × ⎜
⎟
⎝ R1 + R 2 ⎠
Where VFB is the feedback voltage, and VFB=0.925V.
Thus, VOUT can be expressed as:
⎛ R1 + R 2 ⎞
VOUT = 0.925 × ⎜
⎟
⎝ R2 ⎠
First, fix R2 based on the recommended value, 10kΩ.
Then, R1 can be expressed as:
There are two important parameters of the input
capacitor: the voltage rating and RMS current rating.
The voltage rating should be at least 1.25 times
greater than the maximum input voltage, and the
RMS current of input capacitor can be expressed as:
⎛V
⎞
R1 = R 2 × ⎜ OUT − 1⎟
⎝ 0.925 ⎠
I CIN _ RMS = I OUT ( MAX ) ×
3.2 Inductor Setting
The inductor is used to supply smooth current to
output when driven by a switching voltage. Its value
relies on the operating frequency, load current, ripple
current, and duty cycle.
f SW
⎞
⎟⎟
⎠
As indicated by the RMS current equation above,
ICIN_RMS reaches the highest level at the duty cycle of
50%. So the RMS current of input capacitor should
be greater than half of the output current under this
worst case. For reliable operation and best
performance, ceramic capacitors are preferred for
input capacitor because of their low ESR and high
ripple current rating. And X5R or X7R type dielectric
ceramic capacitors are preferred due to their better
temperature and voltage characteristics. Additionally,
when selecting ceramic capacitor, make sure its
capacitance is big enough to provide sufficient charge
to prevent excessive voltage ripple at input. The input
ripple voltage can be approximately expressed as
below:
VIN − VOUT
× VIN × I OUT × 26%
ΔVIN =
Where VIN is the input voltage, IOUT is the output
current, and fSW is the oscillator frequency.
⎛ V
I OUT
× ⎜⎜1 − OUT
f SW × C IN ⎝
VIN
⎞ VOUT
⎟⎟ ×
⎠ VIN
Where ΔVIN is the input ripple voltage.
Another important parameter for the inductor is the
current rating. After fixing the inductor value, the
peak inductor current can be expressed as:
Nov. 2012
⎛ VOUT
⎜⎜1 −
V IN
⎝
Where ICIN_RMS is the RMS current of input capacitor.
A higher-value inductor can decrease the ripple
current and output ripple voltage, however usually
with larger physical size. So some compromise needs
to be made when selecting the inductor. The
peak-to-peak inductor ripple current is 26% of the
maximum output current when operating in
continuous mode (In most applications, a good
compromise is from 20% to 30% of the maximum
load current of the converter), and the inductor L1
can be selected according to:
L1 = VOUT ×
VOUT
V IN
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BCD Semiconductor Manufacturing Limited
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Application Note 1084
The AP3502F/3F employs current-mode control to
achieve easy compensation and fast dynamic
response. Optimal loop compensation depends on the
output capacitor, inductor, load, compensation
network and also the device itself. For a stable system,
the values for the compensation network is shown in
Table 1 and Table 2.
3.4 Output Capacitor Setting
The output capacitor can be selected based upon the
desired output ripple and transient response. The
output voltage ripple depends directly on the ripple
current and is affected by two parameters of the
output capacitor: total capacitance and the Equivalent
Series Resistance (ESR). The output ripple voltage
can be expressed as:
⎡
⎛
⎣
⎝ 8 × COUT × f SW
ΔVO = ΔI L × ⎢ RESR + ⎜⎜
1
⎞⎤
⎟⎟⎥
⎠⎦
VIN/VOUT
(V)
12/1.2
12/1.8
12/2.5
12/3.3
12/5
Where ΔVO is the output ripple voltage, and RESR is
ESR of output capacitor.
VIN/VOUT
(V)
12/1.2
12/1.8
12/2.5
12/3.3
12/5
The output capacitor selection will also affect the
output drop voltage during load transient. The output
drop voltage during load transient is dependent on
many factors. However, an approximation of the
transient drop ignoring loop bandwidth can be
expressed as:
R1
(kΩ)
3
9.53
16.9
26.1
44.1
C3
(nF)
6.8
6.8
5.6
3.3
2.2
R3
(kΩ)
7.5
10
10
13
13
If the VIN/VOUT value of desired solution are not found
from the table above, the loop transfer function
should be analyzed to optimize the loop
compensation. The overall loop transfer function is
the product of the power stage and the feedback
network transfer function. The power stage transfer
function is dictated by the modulator, the output LC
filter and load. The feedback transfer function is
dictated by the error amplifier gain, external
compensation network and feedback resistor ratio.
The purpose of loop compensation is to shape the
loop transfer function in order to meet the desired
loop gain. The crossover frequency should be set
firstly. Because lower crossover frequency may result
in slower line/load transient responses, while higher
crossover frequency may result in system instability.
A good compromise is to set the crossover frequency
below 10% of the switching frequency. The crossover
frequency (fC) can be expressed as below:
L × ΔI TRAN
+
C OUT × (VIN − VOUT )
Where ΔITRAN is the output transient load current step,
and VDROP is the output voltage drop (ignoring loop
bandwidth).
Both the voltage rating and RMS current rating of the
capacitor needs to be carefully examined when
designing a specific output ripple or transient drop.
The output capacitor voltage rating should be greater
than 1.5 times of the maximum output voltage. In the
buck converter, output capacitor current is continuous.
The RMS current is decided by the peak-to-peak
inductor ripple current. It can be expressed as:
ΔI L
12
⎛ G × GCS × R3 VFB
f C = ⎜⎜ EA
×
VOUT
⎝ 2π × C OUT
Where ICOUT_RMS is the RMS current of output
capacitor
3.5 Loop Compensation
Nov. 2012
R3
(kΩ)
7.5
15
13
13
13
Table 2. AP3503F Compensation Value
R-C Combination
2
I COUT _ RMS =
C3
(nF)
4.7
6.8
5.6
4.7
3.3
Table 1. AP3502F Compensation Value
R-C Combination
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R ceramic
dielectric capacitor, or other low ESR tantalum
capacitor or aluminum electrolytic capacitor are
recommended.
V DROP = ΔI TRAN × RESR
R1
(kΩ)
3
9.53
16.9
26.1
44.1
⎞
⎟⎟ < 0.1 × f SW
⎠
Where fC is the crossover frequency, GEA is the error
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Application Note 1084
amplifier voltage gain, GCS is the current sense
trans-conductance. And the desired crossover
frequency can be set via compensation resister R3.
3.6 Bootstrap capacitor
The bootstrap capacitor provided is used to drive the
power switch’s gate above the supply voltage. The
bootstrap capacitor is supplied by an internal 5V
supply and placed between SW pin and BS pin to
form a floating supply across the power switch driver.
So the bootstrap capacitor should be a good quality
and high-frequency ceramic capacitor. For best
performance, the bootstrap capacitor should be X5R
and X7R ceramic capacitor, and is recommended to
be 10nF.
For sufficient phase margin, the loop gain slope
should be -20db/decade at the cross frequency. To
suffice this requirement, the output filter pole (fP_OUT),
which is product by output capacitor and the load
resister, should be cancelled by the zero point of error
amplifier (fZ_EA) due to the compensation capacitor
(C3) and the output resistor of the error amplifier.
They can be expressed as:
f P _ OUT
⎛
1
= ⎜⎜
⎝ 2π × C OUT × ROUT
4. PCB Layout Guidance
⎞
⎟⎟
⎠
PCB layout is an important part for DC-DC converter
design. Poor PCB layout may reduce the converter
performance and disrupt its surrounding circuitry due
to EMI. A good PCB layout should follow guidance
below:
1
⎛
⎞
f Z _ EA = ⎜
⎟
⎝ 2π × C 3 × R3 ⎠
4.1 Power Path Length
The power path of AP3502F/3F includes an input
capacitor, output inductor and output capacitor. Place
them on the same side of PCB and connect them with
thick traces or copper planes on the same layer. The
power components must be kept together closely. The
longer the paths, the more they act as antennas,
radiating unwanted EMI.
Where, fP_OUT is the output filter pole and fZ_EA is the
zero point of error amplifier.
In general, we can set fZ_EA below one-forth of the fC.
So the value of C3 is determined by the following
equation:
C3 >
4.2 Coupling Noise
The external control components should be place as
close to the IC as possible.
4
2π × R3 × f C
R3 and C3 should be set appropriately to make sure
the system work at the desired transient voltage drop
and setting time. If the output capacitor has a large
capacitance and/or a high ESR value, the zero point
resulting from the output capacitor as well as its ESR
should be considered. In this case, the additional
capacitor (C6) should be placed between the COMP
pin and GND. And, C6 can add a pole to the circuit,
thus increasing the mid-frequency width of the
control circuit.
4.3 Feedback Net
Special attention should be paid to the route of the
feedback wring. The feedback trace should be routed
far away from the inductor and noisy power traces.
Try to minimize trace length to the FB pin and
connect feedback network behind the output
capacitors.
4.4 Via Hole
Be careful to the via hole. Via hole will result in high
resistance and inductance to the power path. If heavy
switching current must be routed through via holes
and/or internal planes, use multiple parallel via holes
to reduce their resistance and inductance.
⎞
⎛
1
⎟⎟
f Z _ ESR = ⎜⎜
⎝ 2π × COUT × RESR ⎠
Where fZ_ESR is the zero point of output filter. If
needed, the value of C6 can be expressed as:
C6 =
Typical examples of AP3502F/3F PCB layer are
shown in Figure 3 and 4.
C OUT × RESR
R3
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Application Note 1084
Figure 3. Top Layer
Figure 4. Bottom Layer
Nov. 2012
Rev. 1. 1
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