AN1098

Application Note 1098
Design Consideration with AT5503
Prepared by Cheng Zhi Peng
System Engineering Dept.
1. Introduction
2. General Description
The AT5503 is a current-mode step-down DC-DC
converter, capable of driving a 3A load with high
efficiency, especially high efficiency at light load,
excellent line and load regulation. The AT5503 integrates
cycle-by-cycle current limit protection, programmable
soft-start, hiccup mode for short circuit protection and over
temperature protection, which can notably increase the
system reliability.
The AT5503 is a synchronous step-down converter 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 AT5503 is also a high reliability IC with integrated
OCP, OVP, OTP, UVLO circuit. For more information
please refer to the functional block diagram (Figure 1).
Figure 1. Functional Block Diagram of AT5503
2.1 Programmable Soft-start
The soft-start time of the AT5503 is fully user
programmable by selecting different CSS value. The CSS is
charged by a 5μA 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:
5μA
C SS = t SS ×
GND, tSS is the desired soft-start time and VFB is the
feedback voltage.
2.2 Over Current Protection
The AT5503 has internal over current protection function
to protect itself from catastrophic failure. The AT5503 can
monitor the drain-to-source current of M1. The peak
current-limit threshold is internally set at 5.6A. When the
inductor current is higher than the current limit threshold,
OCP function will be triggered, forcing M1 to turn off, and
this will last until the next switching cycle.
VFB
Where CSS is the required capacitor between SS pin and
Apr. 2013
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
1
Application Note 1098
2.3 Short Circuit Protection
When the circuit is shorted, the output is connected to
GND and FB voltage is lower than 0.3V, reducing the
switching frequency to 180kHz. Meanwhile, the current
flowing through inductor reaches the current limit
threshold of upper-side switch, then the OCP function is
triggered. After that, inductor current will be decreased and
SS pin begins to discharge; when SS discharge voltage
reaches to about 0.2V, the IC enters soft-start mode and
charge to SS pin. Thus, inductor current increases slowly
and triggers the current limit threshold of upper-side switch
again. With that, SS pin discharges and inductor current
decreases once more. This process will be repeated
continually until the system release from SCP (short circuit
protection) function and restart normally, as showed in
Figure 2.
be turned off. The AT5503 will restart once released from
OVP condition.
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 AT5503 will
restart automatically under the control of soft-start circuit
when the junction temperature decreases to 130ºC.
2.6 High Efficiency at Light Load
When the systems work in light load, discontinuous
conduction mode (DCM) is usually more advantage than
continuous conduction mode (CCM). Since there’ll be
higher efficiency in DCM mode than CCM, it can supply
more power if the loss power are same. The AT5503 is
available for DCM, so it can achieve high efficiency at
light load.
VOUT
(2V/div)
100
VSS
(2V/div)
90
80
Efficiency (%)
VSW
(10V/div)
IL
(2A/div)
70
60
50
40
VIN=12V, VOUT=3.3V, L=4.7μH
30
Figure 2. AT5503 Short Circuit Protection and Recovery
20
2.4 Over Voltage Protection
The AT5503 has internal OVP circuits. When VOUT is
higher than the OVP threshold, the power switching will
10
0.1
0.2
0.3
0.4
0.5
Output Current (A)
Figure 3. Efficiency vs. Output Current
Figure 4. Typical Application of AT5503
Apr. 2013
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
2
Application Note 1098
3. Application Information
I PEAK = I OUT +
Typical application circuit is shown in Figure 4. For circuit
parameters setting please refer to the following
descriptions.
(VIN
− VOUT ) × VOUT
2 × V IN × f SW × L
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=0.8V.
Thus, VOUT can be expressed as:
⎛ R1 + R 2 ⎞
VOUT = 0.8 × ⎜
⎟
⎝ R2 ⎠
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:
First, fix R2 based on the recommended value, 10kΩ. Then,
R1 can be expressed as:
VOUT
V IN
⎛ VOUT
⎜⎜1 −
V IN
⎝
⎞
⎟⎟
⎠
⎛V
⎞
R1 = R 2 × ⎜ OUT − 1⎟
⎝ 0.8
⎠
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.
Where ICIN_RMS is the RMS current of input capacitor.
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:
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 L can be selected
according to:
L = VOUT ×
f SW
V IN − VOUT
× V IN × I OUT × 26%
ΔVIN =
⎞ VOUT
⎟⎟ ×
⎠ VIN
Where ΔVIN is the input ripple voltage.
Where VIN is the input voltage, IOUT is the output current,
and fSW is the oscillator frequency.
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).
Another important parameter for the inductor is the current
rating. After fixing the inductor value, the peak inductor
current can be expressed as:
Apr. 2013
⎛ V
I OUT
× ⎜⎜1 − OUT
f SW × C IN ⎝
VIN
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
3
Application Note 1098
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:
The output ripple voltage can be expressed as:
⎡
⎛
⎣
⎝ 8 × COUT × f SW
ΔVO = ΔI L × ⎢ RESR + ⎜⎜
1
⎞⎤
⎟⎟⎥
⎠⎦
Where ΔVO is the output ripple voltage, and RESR is ESR of
output capacitor.
⎛ G × GCS × RC V FB
f C = ⎜⎜ EA
×
VOUT
⎝ 2π × C OUT
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.
Where fC is the crossover frequency, GEA is the error
amplifier transconductance, GCS is the current sense
trans-conductance. And the desired crossover frequency
can be set via compensation resister RC.
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:
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 (CC) and the output resistor of
the error amplifier. They can be expressed as:
L × ΔI TRAN
C OUT × (VIN − VOUT )
2
VDROP = ΔI TRAN × RESR +
Where ΔITRAN is the output transient load current step, and
VDROP is the output voltage drop (ignoring loop
bandwidth).
⎛
1
f P _ OUT = ⎜⎜
2
π
C
×
OUT × ROUT
⎝
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:
I COUT _ RMS =
⎛
1
f Z _ EA = ⎜⎜
⎝ 2π × C C × RC
⎞
⎟⎟
⎠
⎞
⎟⎟
⎠
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 CC is determined by the following equation:
ΔI L
12
CC >
Where ICOUT_RMS is the RMS current of output capacitor.
3.5 Loop Compensation
The AT5503 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.
4
2π × RC × f C
RC and CC 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 (CP) should be placed
between the COMP pin and GND. And, CP can add a pole
to the circuit, thus increasing the mid-frequency width of
the control circuit.
For different VIN/VOUT value, 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
Apr. 2013
⎞
⎟⎟ < 0.1 × f SW
⎠
⎛
⎞
1
⎟⎟
f Z _ ESR = ⎜⎜
⎝ 2π × COUT × RESR ⎠
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
4
Application Note 1098
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 fZ_ESR is the zero point of output filter. If needed, the
value of CP can be expressed as:
CP =
C OUT × RESR
RC
4.2 Coupling Noise
The external control components should be placed as close
to the IC as possible.
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.
4.3 Feedback Net
Special attention should be paid to the route of the
feedback ring. 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.
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 below guidance:
Typical examples of AT5503 PCB layer are shown in
Figure 5, 6.
4.1 Power Path Length
The power path of AT5503 includes an input capacitor,
output inductor and output capacitor. Place them on the
Figure 5. Top Layer
Figure 6. Bottom Layer
5. Recommended Components for Some
Standard Output Voltages
most standard output voltages. The following table lists
recommended components for some standard output
voltages. Listed compensation components (RC, CC) values
are based on the output capacitors installed on these
boards.
The output voltage of these boards is set to 3.3V. The
boards are laid out to accommodate most commonly used
inductors and output capacitors and to be programmed for
Apr. 2013
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
5
Application Note 1098
Part Number
AT5503
VIN/VOUT(V)
5/1.2
5/1.8
5/2.5
5/3.3
12/1.2
12/1.8
12/2.5
12/3.3
12/5.0
R1(kΩ)
5
12.5
21.25
31.25
5
12.5
21.25
31.25
52.5
RC(kΩ)
4.3
5.6
10
10
4.3
6.8
10
13
13
CC(nF)
6.8
6.8
6.8
5.6
6.8
6.8
5.6
3.3
2.2
L(μH)
2.2
2.2
2.2
4.7
2.2
2.2
2.2
4.7
6.8
Table 1. AT5503 Compensation Value R-C Combination
Apr. 2013
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
6
Similar pages