AME5285/Step-Down Converter

AME
AME5285
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
n General Description
n Applications
l TV
The AME5285 is a Synchronous Rectified Step-Down
Converter with internal power MOSFETs. It achieves 5A
continous output current over a wide switching frequency
range with excellent load and line regulation.
Current mode operation provides fast transient response
and eases of loop stabilization. Internal soft-start minimizes the inrush supply current at startup. The circuit
protection includes cycle-by cycle current limiting, output short circuit frequency protection and thermal shutdown.
This device is available in SOP-8/PP package with exposed pad for low thermal resistance.
l Distributed Power Systems
l Pre-Regulator for Linear Regulators
n Typical Application
L1
1.5µH
VIN
5V
CIN
10µF
IN
OFF ON
SW
EN
AME5285
R1
6K
VOUT
1V 5A
COUT
22µF
FB
COMP
n Features
l 5A Output Current
C2
Optional
C1
680pF
R3
5.1K
GND
FREQ
R2
24K
RFREQ
30K
l 55mΩ/45mΩ Internal Power MOSFET Switch
l Stable with Low ESR Output Ceramic Capacit
-ors
l Up to 95% Efficiency
l Less than 10µA Shutdown Current
l Wide Switching Frequency Range from
300KHz~1.4MHz
l Thermal Shutdown
l Cycle by cycle Over Current Protection and
Hiccup
l Output Adjustable from 0.8V to VIN
l Short Circuit Frequency Protection
l Green Products Meet RoHS Standards
Rev. B.01
1
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Functional Block Diagram
IN
CURRENT
SENSE
EN
ENABLE
CURRENT
LIMIT
UVLO
FREQ
OSC
SW
SLOPE
+
+
COMP
SOFT
START
EA
-
GND
+
0.8V
VREF
-
LOGIC
+
OVP
2
-
0.9V
SW
PWM
OTP
FB
DRIVER
IRCMP
+
PGND
Rev. B.01
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Pin Configuration
SOP-8/PP
Top View
8
7
6
AME5285-AZAxxx
5
1. COMP
2. GND
3. EN
AME5285
4. IN
5. SW
1
2
3
6. SW
4
7. FREQ
8. FB
* Die Attach:
Conductive Epoxy
Note: Connect exposed pad (heat sink on the back) to GND.
n Pin Description
Pin Number Pin Name
Rev. B.01
Pin Description
Compensation Node. COMP is used to compensate the regulation control
loop. Connect a series RC network from COMP to GND to compensate the
regulation control loop. In some cases, an additional capacitor from COMP
to GND is required.
1
COMP
2
GND
3
EN
Enable. Internal pull high with a resistor. Pull EN below 0.6V to shut down
the regulator.
4
IN
Power Input. IN supplies the power to the IC, as well as the step-down
converter switches. Bypass IN to GND with a suitable large capacitor to
eliminate noise on the input to the IC.
5, 6
SW
7
FREQ
8
FB
Ground. Connect the exposed pad to GND.
Power Switching Output. SW is the switching node that supplies power to
the output. Connect the output LC filter from SW to the output load. Note
that a capacitor is required from SW to BS to power the high-side switch.
Frequency Adjust Pin. Add a resistor from this pin to ground determines the
switching frequency.
Feedback Input. FB senses the output voltage to regulate that voltage.
Drive FB with a resistive voltage divider from the output voltage. The
feedback reference voltage is 0.8V.
3
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Ordering Information
AME5285 - x x x xxx
Output Voltage
Number of Pins
Package Type
Pin Configuration
Pin Configuration
A
(SOP-8/PP)
4
1. COMP
2. GND
3. EN
4. IN
5. SW
6. SW
7. FREQ
8. FB
Package
Type
Z: SOP/PP
Number of
Pins
A: 8
Output Voltage
ADJ: Adjustable
Rev. B.01
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Absolute Maximum Ratings
Parameter
Maximum
Unit
Supply Voltage
-0.3V to +6V
V
Switch voltage
-0.7V to V IN +0.7V
V
EN, FB, COMP, FREQ to GND
-0.3V to V IN +0.3V
V
HBM
2
kV
MM
200
V
Symbol
Rating
Unit
Ambient Temperature Range
TA
-40 to +85
Junction Temperature Range
TJ
-40 to +125
Storage Temperature Range
T STG
-65 to +150
ESD Classification
n Recommended Operating Conditions
Parameter
o
C
n Thermal Information
Parameter
Package
Die Attach
Thermal Resistance*
(Junction to Case)
Thermal Resistance
(Junction to Ambient)
Symbol Maximum
θ JC
Unit
15
o
SOP-8/PP
Internal Power Dissipation
Conductive Epoxy
θJA
75
PD
1.333
C/W
mW
Maximum Junction Temperature
150
o
C
Lead Temperature (Soldering 10Sec)**
260
o
C
* Measure θJC on backside center of Exposed Pad.
** MIL-STD-202G 210F
Rev. B.01
5
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Electrical Specifications
VIN=5V, TA=25oC, unless otherwise noted.
Parameter
Symbol
Test Condition
Min
Input Voltage Range
3
Input UVLO
2
VEN=5V
Quiescent Current
(No Switching)
Shutdown Current
ISHDN
VEN=0V
Feedback Voltage
VFB
0.784
Feedback Current
I FB
-50
Typ
2.3
Units
5.5
V
2.6
V
450
µA
10
µA
0.8
0.816
V
50
nA
Load Regulation
0A<IOUT<5A
0.25
%
Line Regulation
3.3V<VIN<5.5V
0.25
%/V
EN Voltage High
1.5
V EN
V
EN Voltage Low
EN Leakage Current
Switching Frequency
Short-Circuit Frequency
0.4
IENLK
FSW
V
VEN=3V
4
µA
RFREQ=NC
300
KHz
RFREQ =120KΩ
600
KHz
RFREQ=47K Ω
1
MHz
RFREQ=30K Ω
1.4
MHz
0.25
FSW
8.5
A
-2
A
FSWSC
High-side Switch Current Limit
6
Low-side Switch Current Limit
Maximum Duty Cycle
100
Minimum On Time
%
100
ns
Error Amp Voltage Gain
AEA
600
V/V
Error Amp Tranconductance
GEA
390
µ A/V
0.1
µA
Switch Leakage Current
I SWLK
VSW =0V, VEN=0V
High-side Switch On Resistance
RDSON,HI
55
mΩ
Low-side Switch On Resistance
RDSON,LO
45
mΩ
Thermal Shutdown Protection
OTP
OTH
6
Max
Rising
Hysteresis
170
o
20
o
C
C
Rev. B.01
AME
AME5285
n Detailed Description
Normal Operation
The AME5285 uses a user adjustable frequency, current mode step-down architecture with internal MOSFET
switch. During normal operation, the internal high-side
(PMOS) switch is turned on each cycle when the oscillator sets the SR latch, and turned off when the comparator resets the SR latch. The peak inductor current at
which comparator resets the SR latch is controlled by
the output of error amplifier EA. While the high-side switch
is off, the low-side switch turns on until either the lowside current limit reached or the beginning of the next
switching cycle.
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
Short Circuit Protection
When the output is shorted to ground, the frequency of
the oscillator is reduced to about 1/4 of the normal frequency to ensure that the inductor current has more time
to decay, thereby preventing runaway. Meanwhile,
AME5285 enters hiccup mode, the average short circuit
current is greatly reduced to alleviate the thermal issue
and to protect the regulator.
Dropout Operation
The output voltage is dropped from the input supply for
the voltage which across the high-side switch. As the
input supply voltage decreases to a value approaching
the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage forces the high-side switch to remain on for more
than one cycle until it reaches 100% duty cycle.
Soft-Start
The AME5285 employs internal soft-start circuitry to
reduce supply inrush current during startup conditions.
Over Temperature Protection
The In most applications the AME5285 does not dissipate much heat due to high efficiency. But, in applications where the AME5285 is running at high ambient temperature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximum junction temperature of the part. If the junction temperature reaches approximately 170oC, the internal high-side power switch will be turned off and the SW
switch will become high impedance.
Rev. B.01
7
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Application Information
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 2.2µH to 4.7µH. Its value is chosen based
on the desired ripple current. Large value inductors lower
ripple current and small value inductors result in higher
ripple currents. Higher V IN or VOUT also increase the ripple
current ∆IL:
 V
1
∆I L =
VOUT 1 − OUT
f ×L
VIN




Capacitor Selection
In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle VOUT/VIN. To
prevent large voltage transients, a low ESR input capacitor sized for maximum RMS current must be used. The
maximum RMS capacitor current is given by:
≅ I OMAX
When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage characteristics of all the ceramics for given value and size.
Output Voltage Programming
The output voltage of the AME5285 is set by a resistive
divider according to the following formula:
A reasonable inductor current ripple is usually set as
1/3 to 1/5 of maximum out current. The DC current rating
of the inductor should be at least equal to the maximum
load current plus half the ripple current to prevent core
saturation. For better efficiency, choose a low DCR inductor.
CIN requires IRMS
For a fixed output voltage, the output ripple is highest
at maximum input voltage since ∆IL increases with input
voltage.
VOUT (V IN − VOUT )
VIN
R1 

V OUT = 0 .8 × 1 +
Volt .
R 2 

Some standard value of R1, R2 for most commonly used
output voltage values are listed in Table 1.
VOUT(V)
R1(KΩ)
R2(KΩ )
1.1
7.5
20
1.2
10
20
1.5
17.4
20
1.8
30
24
2.5
51
24
3.3
75
24
This formula has a maximum at VIN =2V OUT, where
IRMS=IOUT/2. For simplification, use an input capacitor with
a RMS current rating greater than half of the maximum
load current.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating
generally far exceeds the IRIPPLE(P-P) requirement. The
output ripple ∆VOUT is determined by:

1
∆VOUT ≅ ∆I L  ESR +
8 fCOUT

8



Rev. B.01
AME
AME5285
Loop Compensation
The AME5285 employs peak current mode control for
easy use and fast transient response. Peak current mode
control eliminates the double pole effect of the output LC filter. It greatly simplifies the compensation loop design.
With peak current mode control, the buck power stage
can be simplified to be a one-pole and one-zero system
in frequency domain. The pole can be calculated by:
f P1 =
1
2π × C OUT × R L
The zero is a ESR zero due to output capacitor and its
ESR. It can be calculated by:
f Z1 =
1
2π × C OUT × ESRCOUT
Where COUT is the output capacitor, RL is load resistance; ESRCOUT is the equivalent series resistance of
output capacitor.
The compensation design is to shape the converter close
loop transfer function to get desired gain and phase. For
most cases, a series capacitor and resistor network connected to the COMP pin sets the pole-zero and is adequate for a stable high-bandwidth control loop.
In the AME5285, FB pin and COMP pin are the inverting
input and the output of internal transconductance error
amplifier (EA). A series RC and CC compensation network connected to COMP pin provides one pole and one
zero:
for RC<<AEA/GEA
f P2 =
1
GEA
≈

A  2π × CC × AEA
2π × CC ×  RC + EA 
G EA 

fZ2 =
1
2 π × C C × RC
Rev. B.01
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
where GEA is the error amplifier transconductance
AEA is the error amplifier voltage gain
RC is the compensation resistor
CC is the compensation capacitor
The desired crossover frequency fC of the system is defined to be the frequency where the control loop has unity
gain. It is also called the bandwidth of the converter. In
general, a higher bandwidth means faster response to
load transient. However, the bandwidth should not be too
high because of system stability concern. When designing the compensation loop, converter stability under
all line and load condition must be considered. Usually,
it is recommended to set the bandwidth to be less than
1/10 of switching frequency. Using selected crossover
frequency, fC, to calculate RC:
RC = f C ×
VOUT 2π × COUT
×
VFB GEA × GCS
where GCS is the current sense circuit transconductance.
The compensation capacitor CC and resistor RC together
make zero. This zero is put somewhere close to the
pole fP1 of selected frequency. CC is selected by:
CC =
COUT × RL
RC
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V OUT immediately shifts by an amount
equal to (∆ILOAD × ESR), where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or
discharge COUT, which generates a feedback error signal.
The regulator loop then acts to return VOUT to its steadystate value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem.
9
AME
AME5285
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
Efficiency Considerations
Although all dissipative elements in the circuit produce
losses, one major source usually account for most of the
losses in AME5285 circuits: I2R losses. The I2R loss
dominates the efficiency loss at medium to high load
currents.
The I2R losses are calculated from the resistances of
the internal switches, RSW, and external inductor RL. In
continuous mode, the average output current flowing
through inductor L is "chopped" between the main switch
and the synchronous switch. Thus the series resistance
looking into the SW pin is a function of both top and
bottom MOSFET RDS(ON) and the duty cycle (D) as follows:
RSW = (RDS(ON)TOP)(D) + (RDS(ON)BOTTOM )(1-D)
The RDS(ON) for both the top and bottom MOSFETs can
be obtained from Electrical Characteristics table. Thus,
to obtained I2R losses, simply add RSW to RL and multiply the result by the square of the average output current.
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for
less than 2% total additional loss.
Thermal Considerations
In most application the AME5285 does not dissipate
much heat due to its high efficiency. But, in applications
where the AME5285 is running at high ambient temperature with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction
temperature reaches approximately 170oC, both power
switches will be turned off and the SW switch will become high impedance.
10
Rev. B.01
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Typical Operating Circuit
VIN
3V to 5V
CIN
10µF
L
1
3
Chip Enable
4
SW
IN
EN
R1
AME5285
COMP
7
FREQ
R3
2
RFREQ
GND
COUT
8
FB
C1
C2
Optional
VOUT
5,6
GND
R2
9 (Exposed pad)
V OUT(V)
CIN (µF)
R1(KΩ )
R2(KΩ )
R3(KΩ )
C1(pF)
L(µH)
COUT(µF )
3.3
10
75
24
25
680
2.2
22
2.5
10
51
24
20
680
2.2
22
1.8
10
30
24
15
680
1.5
22
1.5
10
21
24
13
680
1.5
22
1.2
10
12
24
11
680
1.5
22
1.1
10
6
24
8.2
680
1.5
22
Table 1. Recommended Components Selectin for fsw = 1.4MHz
The ground area must provide adequate heat
dissipating area to the thermal pad andusing
multiple vias to help thermal dissipation.
R3
Connect the
FB pin directly
to feedback
R2 resistors.
C1
COMP 1
8
FB
V OUT
R1
GND
2
GND
7 FREQ
RFREQ
GND
EN
3
6
SW
V IN
4
5
SW
SW
V IN
SW pad should be
connected together to
Inductor by wide and short
trace, keep sensitive
components away from
this trace.
L1
C IN must be placed
between V IN and
GND as close as
CIN
possible
Place the input and output
capacitors as close to the
IC as possible
COUT
V OUT
Figure 3. AME5285 Regulators Layout Diagram
Rev. B.01
11
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Characterization Curve
Efficiency vs. Output Current
Efficiency vs. Output Current
100
100
90
90
VOUT=3.3V
80
70
Effici ency(%)
Efficiency(%)
80
60
50
40
30
VOUT =1V
60
50
40
30
VIN =5V
RFREQ= 30K
20
20
10
0
70
VNI =5V
RFREQ= 30 K
10
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0
5.0
0
0 .5
1 .0
Output Current(A)
3. 0
3. 5
4.0
4. 5
5.0
100
90
90
VOUT=3.3V
80
Efficiency(%)
80
Efficiency(%)
2 .5
Efficiency vs. Output Current
100
70
60
50
40
70
V OUT=1V
60
50
40
30
30
20
20
VIN =5V
R FREQ=NC
10
VIN =5V
RFREQ=NC
10
0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Output Current(A)
12
2 .0
Output Current(A)
Efficiency vs. Output Current
0
1.5
4.0
4.5
5.0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Output Current(A)
Rev. B.01
AME
AME5285
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
n Characterization Curve (Contd.)
Load Step
Load Step
1
1
2
2
Time (100µSec/DIV)
Time (100µSec/DIV)
VIN = 5V
VOUT = 3.3V
IOUT = 1mA to 5A
R
=30K
VIN = 5V
VOUT = 1V
IOUT = 1mA to 5A
R
=30K
FREQ
FREQ
1) VOUT = 100mV/div
2) IL = 2A/div
1) VOUT = 50mV/div
2) IL = 2A/div
Power ON from EN
Power ON form VIN
1
1
2
2
3
3
Time (2.0ms /DIV)
VIN = 5V
VOUT = 3.3V
R
=NC
FREQ
1) VEN = 2V/div
2) VOUT = 2V/div
3) IL = 5A/div
Rev. B.01
Time (5.0ms /DIV)
VIN = 5V
VOUT = 3.3V
R
=NC
FREQ
1) VIN = 2V/div
2) VOUT = 2V/div
3) IL = 5A/div
13
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Characterization Curve (Contd.)
Power Off from EN
Power Off from VIN
1
1
2
2
3
3
Time (5.0ms /DIV)
Time (5.0ms / DIV)
VIN = 5V
VOUT = 3.3V
R
=NC
VIN = 5V
VOUT = 3.3V
R
=NC
FREQ
FREQ
1) VEN = 2V/div
2) VOUT = 2V/div
3) IL = 5A/div
1) VIN = 2V/div
2) VOUT = 2V/div
3) IL = 5A/div
Short Circuit Test
Quiescent Current vs. Input Voltage
490
1
V IN =5V
V OUT =3.3V
2
Quiescent Current (µA)
480
470
460
450
440
430
420
410
V OUT=3.3V
RFREQ=N C
400
3
390
Time (100ms/DIV)
380
3.5
4.0
4.5
5.0
Input Voltage(V)
VIN = 5V
VOUT = 3.3V
R
=NC
FREQ
1) VIN = 5V/div
2) VOUT = 2V/div
3) IL = 5A/div
14
Rev. B.01
AME
5A, 300KHz ~ 1.4MHz Synchronous
Rectified Step-Down Converter
AME5285
n Tape and Reel Dimension
SOP-8PP
P
PIN 1
W
AME
AME
Carrier Tape, Number of Components Per Reel and Reel Size
Package
Carrier Width (W)
Pitch (P)
Part Per Full Reel
Reel Size
SOP-8/PP
12.0±0.1 mm
4.0±0.1 mm
2500pcs
330±1 mm
n Package Dimension
SOP-8PP
TOP VIEW
SIDE VIEW
D1
θ
E1
E2
E
L1
C
PIN 1
D
A1
Rev. B.01
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
A
1.350
1.750
0.053
0.069
A1
0.000
0.150
0.000
0.006
A2
1.350
1.600
0.053
0.063
C
0.100
0.250
0.004
0.010
E
3.750
4.150
0.148
0.163
E1
5.700
6.300
0.224
0.248
L1
0.300
1.270
0.012
0.050
b
0.310
0.510
0.012
0.020
D
4.720
5.120
0.186
0.202
1.270 BSC
e
e
FRONT VIEW
A
A2
b
SYMBOLS
0.050 BSC
θ
0
E2
2.150
2.513
0.085
0.099
D1
2.150
3.402
0.085
0.134
o
8
o
0
o
o
8
15
www.ame.com.tw
E-Mail: sales@ame.com.tw
Life Support Policy:
These products of AME, Inc. are not authorized for use as critical components in life-support
devices or systems, without the express written approval of the president
of AME, Inc.
AME, Inc. reserves the right to make changes in the circuitry and specifications of its devices and
advises its customers to obtain the latest version of relevant information.
 AME, Inc. , November 2012
Document: A005-DS5285-B.01
Corporate Headquarter
AME, Inc.
8F, 12, WenHu St., Nei-Hu
Taipei 114, Taiwan .
Tel: 886 2 2627-8687
Fax: 886 2 2659-2989