Micro Linear ML4863IS High efficiency flyback controller Datasheet

G
FEATURINperature Range
July 2000
Tem
ommercial
Extended C
70˚C
to
C
0˚
-2
ment
dheld Equip
an
H
e
bl
ta
or
for P
ML4863*
High Efficiency Flyback Controller
GENERAL DESCRIPTION
FEATURES
The ML4863 is a flyback controller designed for use in
multi-cell battery powered systems such as PDAs and
notebook computers. The flyback topology is ideal for
systems where the battery voltage can be either above or
below the output voltage, and where multiple output
voltages are required.
■
Variable frequency current mode control and
synchronous rectification for high efficiency
■
Minimum external components
■
Guaranteed start-up and operation over a wide input
voltage range (3.15V to 15V)
■
High frequency operation (>200kHz) minimizes the
size of the magnetics
■
Flyback topology allows multiple outputs in addition to
the regulated 5V
■
Built-in overvoltage and current limit protection
The ML4863 uses the output voltage as the feedback
control signal to the current mode variable frequency
flyback controller. In addition, a synchronous rectifier
control output is supplied to provide the highest possible
conversion efficiency (greater than 85% efficiency over a
1mA to 1A load range).
The ML4863 has been designed to operate with a
minimum number of external components to optimize
space and cost.
*Some Packages Are Obsolete
BLOCK DIAGRAM
VCC
SHDN
3
1
4
BIAS & UVLO
VIN
VFB
VCC
4.5V
LDO
5
VFB
VFB
–
+
I
COMP
–
GND
8
VCC
CURRENT
COMPARATOR
+
VREF
SWITCHING
CONTROL
OUT 1
A1
6
18mV
Rgm
18mV
VCC
RECTIFIER
COMPARATOR
OUT 2
–
COMP
+
CROSS-CONDUCTION
PROTECTION
BLANKING
A2
7
SENSE
2
1
ML4863
PIN CONFIGURATION
ML4863
8-Pin SOIC (S08)
VIN
1
8
GND
SENSE
2
7
OUT 2
SHDN
3
6
OUT 1
VFB
4
5
VCC
TOP VIEW
PIN DESCRIPTION
PIN
NAME
FUNCTION
1
VIN
Battery input voltage
2
SENSE
Secondary side current sense
3
SHDN
Pulling this pin high initiates a
shutdown mode to minimize battery
drain
4
2
VFB
Feedback input from transformer
secondary, and supply voltage when
VOUT > 4.5V
PIN
NAME
FUNCTION
5
VCC
Internal power supply node for
connection of a bypass capacitor
6
OUT 1
Flyback primary switch MOSFET driver
output
7
OUT 2
Flyback synchronous rectifier MOSFET
driver output
8
GND
Analog signal ground
ML4863
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
Lead Temperature (Soldering 10 Sec.) ..................... 260ºC
Thermal Resistance (qJA) .................................... 160ºC/W
VIN ................................................................. GND – 0.3V to 18V
Voltage on any other pin ........................... GND – 0.3V to 7V
Source or Sink Current (OUT1 & OUT2) ...................... 1A
Junction Temperature .............................................. 150ºC
Storage Temperature Range...................... –65ºC to 150ºC
Temperature Range
ML4863CS ................................................. 0ºC to 70ºC
ML4863ES ............................................. –20ºC to 70ºC
ML4863IS .............................................. –40ºC to 85ºC
VIN Operating Range ................................... 3.15V to 15V
OPERATING CONDITIONS
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = 12V, TA = Operating Temperature Range (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
C Suffix
2.1
2.5
2.8
µs
E/I Suffix
2.1
2.5
2.95
µs
VFB = 0V
450
650
850
ns
Line, Load, & Temp
4.85
5
5.15
V
OSCILLATOR
tON
ON Time
Minimum Off Time
VFB REGULATION
Total Variation
OUTPUT DRIVERS
OUT1 Rise Time
CLOAD = 3nF, 20% to 90% of VCC
60
70
ns
OUT1 Fall Time
CLOAD = 3nF, 90% to 20% of VCC
60
70
ns
OUT2 Rise Time
CLOAD = 3nF, 20% to 90% of VCC
60
70
ns
OUT2 Fall Time
Continuous Mode, CLOAD = 3nF,
90% to 20% of VCC
60
70
ns
Discontinuous Mode, CLOAD = 3nF,
90% to 20% of VCC
125
150
ns
SHDN
Input High Voltage
2.0
V
Input Low Voltage
Input Bias Current
SHDN = 5V
SENSE Threshold – Full Load
VIN = 5V, VFB = VFB (No Load) – 100mV
SENSE Threshold – Short Circuit
VFB = 0V
0.8
V
5
10
µA
150
160
mV
235
mV
SENSE
130
CIRCUIT PROTECTION
Undervoltage Lockout Start-up Threshold
3.0
3.15
V
Undervoltage Lockout Hysteresis
0.5
0.6
V
3
ML4863
ELECTRICAL CHARACTERISTICS
SYMBOL
(Continued)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
100
150
µA
SHDN = 5V
20
25
µA
SHDN = 5V, VIN < 6V
5
10
µA
SUPPLY
IFB
VFB Quiescent Current
IIN
VIN Shutdown Current
VCC
Note 1:
4
VCC Output Voltage
VFB = 0V, VIN = 15V, CVCC = 0.1µF
4.5
5.5
V
VFB = 0V, VIN = 6V, CVCC = 0.1µF
4.0
5.0
V
VFB = 0V, VIN = 3.15V, CVCC = 0.1µF
2.8
VFB = 5V
4.5
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
V
5
5.15
V
ML4863
FUNCTIONAL DESCRIPTION
TRANSCONDUCTANCE AMPLIFIER
The ML4863 utilizes a flyback topology with constant ontime control. The circuit determines the length of the offtime by waiting for the inductor current to drop to a level
determined by the feedback voltage (VFB). Consequently,
the current programming is somewhat unconventional
because the valley of the current ripple is programmed
instead of the peak. The controller automatically enters
burst mode when the programmed current falls below
zero. Constant on-time control therefore features a
transition into and out of burst mode which does not
require additional control circuitry.
The control circuit is made up of four distinctive blocks;
the constant on-time oscillator, the current programming
comparator, the feedback transconductance amplifier, and
the synchronous rectifier controller. A simplified circuit
diagram is shown in Figure 1.
OSCILLATOR & COMPARATOR
The oscillator has a constant on-time and a minimum offtime. The off-time is extended as long as the output of the
current programming comparator is low. Note that in
constant on-time control, a discharge (off-time) cycle is
needed for the inductor current to be sensed. The
minimum off-time is required to account for the finite
circuit delays in sensing the inductor output current.
The feedback transconductance amplifier generates a
current from the voltage difference between the output
and the reference. This current produces a voltage across
Rgm that adds to the negative voltage on the current sense
resistor, RSENSE. When the current level in the inductor
drops low enough to cause the voltage at the non-inverting
input of the current programming comparator to go
positive, the comparator trips and the converter starts a
new on-cycle. The current programming comparator
controls the length of the off-time by waiting until the
current in the secondary decreases to the value specified
by the feedback transconductance amplifier.
In this way, the feedback transconductance amplifier‘s
output current steers the current level in the inductor.
When the output voltage drops due to a load increase, it
will increase the output current of the feedback amplifier
and generate a larger voltage across Rgm which in turn
raises the secondary current trip level. However, when the
output voltage is too high, the feedback amplifier’s output
current will eventually become negative. Because the
output current of the inductor can never go negative by
virtue of the diode, the non-inverting input of the
comparator will also stay negative. This causes the
converter to stop operation until the output voltage drops
enough to increase the output current of the feedback
transconductance amplifier above zero.
VOUT
IS
VIN
RESR
4
LP
FEEDBACK
TRANSCONDUCTANCE
AMPLIFIER
RP
+
CP
VREF
CURRENT
PROGRAMMING
COMPARATOR
CONSTANT ON-TIME
MINIMUM OFF-TIME
OSCILLATOR
C
ONE SHOT
tON
2.5µs
+
–
1:1
LOAD
VFB
COMP
–
OUT 1
6
ONE SHOT
tOFF
500ns
Rgm
RECTIFIER
COMPARATOR
–
COMP
+
OUT 2
BLANKING
7
A2
SENSE
ML4863
2
RSENSE
Figure 1. Schematic of the ML4863 Controller and Power Stage
5
ML4863
FUNCTIONAL DESCRIPTION
(Continued)
SYNCHRONOUS RECTIFIER CONTROL
where h = converter efficiency.
The control circuitry for the synchronous rectifier does not
influence the operation of the main controller. The
synchronous rectifier is turned on during the minimum off
time, or whenever the SENSE pin is less than –18mV.
During transitions where the primary switch is turned on
before the voltage on the SENSE pin goes above –18mV,
the gate of the synchronous rectifier is discharged softly to
avoid accidently triggering the current-mode comparator
with the gate discharge spike on the sense resistor.
Once RSENSE has been determined, LP can be found:
The part will also operate with a Schottky diode in place
of the synchronous rectifier, but the conversion efficiency
will suffer.
The normal operating range and current limit point are
determined by the current programming comparator. They
are dependent on the value of the synchronous rectifier
current sense resistor (RSENSE), the nominal transformer
primary inductance (LP), and the input voltage.
RSENSE can be calculated by:
6
VIN0 MIN5
VOUT + VIN
´
150mV
V
I 0 5 + 20 ´ V 0 0 5 ´ I5
IN MIN
OUT MAX
IN MAX
0
OUT MAX
(2)
Three operational modes are defined by the voltage at the
SENSE pin at the end of the off-time: discontinuous mode,
continuous mode, and current limit. The following values
can be used to determine the current levels of each mode:
VSENSE < 0V: discontinuous mode
0V < VSENSE < 160mV: continuous mode
160mV < VSENSE < 235mV: current limit
CURRENT LIMIT AND MODES OF OPERATION
R SENSE =
LP = (25 × 10 −6 ) × VIN0MAX5 × R SENSE
´h
5
(1)
Inserting the maximum value of VSENSE for each
operational mode into the following equation will
determine the maximum current levels for each
operational mode:
IOUT =
VIN
V
t
× VIN
× SENSE + ON
×η
VOUT + VIN
R SENSE
2 × LP
(3)
ML4863
DESIGN CONSIDERATIONS
DESIGN PROCEDURE
See Table 1 for suggested component manufacturers.
A typical design can be implemented by using the
following procedure.
Component Manufacturer
1.
Sense
Resistors
Dale
IRC
LRC Series
WSL Series
(402) 563-6506
(512) 992-7900
The maximum input voltage (VIN(MAX))
The mainimum input voltage (VIN(MIN))
The maximum output current (IOUT(MAX))
The maximum output ripple (DVOUT)
Inductors
Coilcraft
R4999
(847) 639-6400
As a general design rule, the output ripple should be kept
below 100mV to ensure stability.
Capacitors
Specify the application by defining:
MOSFETs
2.
Select a sense resistor, RSENSE, using equation 1.
3a.
Determine the inductance required for the
optimum output ripple using equation 2.
3b.
Determine the minimum inductor current rating
required. The peak inductor current is calculated
using the following formula:
IL PEAK =
. ™ 10 -6 )
235mV VIN ( MAX) ™ (25
+
R SENSE
LP
(4)
3c.
Specify the inductor's DC winding resistance. A
good rule of thumb is to allow 5mW, or less, of
resistance per µH of inductance. For minimum
core loss, choose a high frequency core material
such as Kool-Mu, ferrite, or MPP.
3d.
Specify the coupled inductor's turns ratio:
C = IOUT ( MAX)
4b.
+ VIN ( MAX)
VOUT
™ 25. ™ 10
DV
(207) 282-5111
Sprague
593D Series
(207) 324-4140
National
NDS94XX
NDS99XX
(800) 272-9954
Motorola
MMDF Series
MMSF Series
(602) 897-5056
Siliconix
Littlefoot Series
(408) 988-8000
Select the sense resistor, RSENSE, using Equation 1:
4
150mV
4V
×
+
× 0.85
5+ 4
500mA 20 × 6 × 0.5
(1a)
Determine the inductance required using
equation 2.
LP = (25 × 10 −6 ) × 6 × 0.12 = 18µH
3b.
(2a)
Determine the minimum inductor current rating
required.
(6)
As a final design check, evaluate the system
stability (no compensation, single pole response)
by using the following equation:
R
!
TPS series
2.
(5)
150mV
∆VOUT ≤ (6 × 10 −6 ) ×
AVX
Specify the application by defining:
VIN(MAX) = 6V
VIN(MIN) = 4V
IOUT(MAX) = 500mA
DVOUT = 100mV
3a.
DVOUT ™ R SENSE
LPE-6562 Series (605) 665-9301
LPT-4545 series
RSENSE = 138mW @ 120mW
-6
OUT
OCTA-PAC Series (561)241-7876
Dale
1.
R SENSE =
Establish the maximum allowable ESR for the
ouput capacitor:
RESR <
5.
OUT
Coiltronics
DESIGN EXAMPLE
Calculate the minimum output capacitance
required using:
V
™
Phone
Table 1. Component Suppliers
Np : Ns = 1:1
4a.
Part
Number
SENSE
× (VOUT + VIN (MIN) )
LP
"#
$
IL PEAK
235mV 6 × (25
. × 10 −6 )
=
+
= 2.79A
120mΩ
18 × 10 –6
(4a)
(7)
where RSENSE and LP are the actual values to be
used.
7
ML4863
DESIGN CONSIDERATIONS (Continued)
3c.
3d.
Specify the inductor’s DC winding resistance:
LAYOUT
LDCR = 90mW
Good PC board layout practices will ensure the proper
operation of the ML4863. Important layout considerations
follow:
Specify the coupled inductor's turn ratio:
Np : Ns = 1:1
4a.
• The connection from the current sense resistor to the
SENSE pin of the ML4863 should be made by a
separate trace and connected right at the sense resistor
lead.
Calculate the minimum output capacitance
required using equation 5.
C = 0.50 ×
4b.
5 + 6 × 25. × 10
5 0.1
−6
= 55µF
(5a)
Establish the maximum ESR for the output
capacitor using equation 6.
RESR <
• Trace lengths from the capacitors to the inductor, and
from the inductor to the FET should be as short as
possible to minimize noise and ground bounce.
0.1× 0.12
= 80mW
150mV
(6a)
Based on these calculations, the design should use two
100µF capacitors, with an ESR of 100mW each, in parallel
to meet the capacitance and ESR requirements.
5.
• The VCC bypass capacitor needs to be located close to
the ML4863 for adequate filtering of the IC's internal
bias voltage.
• Power and ground planes must be large enough to
handle the current the converter has been designed for.
See Figure 5 for a sample PC board layout.
As a final design check, evaluate the system
stability using equation 7.
100mV ≤ (6 × 10 −6 ) ×
0.12 × (5 + 4) "# = 360mV (7a)
! 18 × 10 $
–6
Since the inequality is met, the circuit should be stable.
Some typical application circuits are shown in Figures 2, 3,
and 4.
VOUT
5V, 1A
400µF
Coiltronics
CTX20-4
VIN
47µF
VIN
100µF
ML4863
VIN
ML4863
NDS9955
GND
VIN
GND
SENSE OUT 2
SENSE OUT 2
SHDN OUT 1
SHDN OUT 1
VFB
VFB
VCC
1µF
VCC
NDS9410
NDS9410
1µF
100mΩ
Figure 2. 5V, 1A Circuit
8
VOUT
5V, 2A
800µF
Dale
LPE6562
50mΩ
Figure 3. 5V, 2A Circuit
ML4863
12V
C4
33µF
20V
C5
33µF
20V
5V
C6
100µF
6.3V
T1
DALE
LPE-6562-A145 7
9
8
2
3
C7
100µF
6.3V
C8
100µF
6.3V
C9
100µF
6.3V
C10
100µF
6.3V
C11
100µF
6.3V
C12
100µF
6.3V
3.3V
1,5
C13
100µF
6.3V
6,10
NDS9955
Q1A
Q1B
4
Q2A
Q2B
MMDF3N03
ML4863
VIN
SHDN
C1
33µF
20V
C2
33µF
20V
VIN
R1
120mΩ
GND
SENSE OUT 2
R2
30mΩ
SHDN OUT 1
VFB
VCC
C3
1µF
50V
R3
60mΩ
Figure 4. 5W Multiple Output DC-DC Converter
Figure 5. Typical PC Board Layout
9
ML4863
PHYSICAL DIMENSIONS inches (millimeters)
Package: S08
8-Pin SOIC
0.189 - 0.199
(4.80 - 5.06)
8
PIN 1 ID
0.148 - 0.158 0.228 - 0.244
(3.76 - 4.01) (5.79 - 6.20)
1
0.017 - 0.027
(0.43 - 0.69)
(4 PLACES)
0.050 BSC
(1.27 BSC)
0.059 - 0.069
(1.49 - 1.75)
0º - 8º
0.055 - 0.061
(1.40 - 1.55)
0.012 - 0.020
(0.30 - 0.51)
0.004 - 0.010
(0.10 - 0.26)
0.015 - 0.035
(0.38 - 0.89)
0.006 - 0.010
(0.15 - 0.26)
SEATING PLANE
ORDERING INFORMATION
PART NUMBER
TEMPERATURE RANGE
PACKAGE
ML4863CS
ML4863ES
ML4863IS (Obsolete)
0ºC to 70ºC
–20ºC to 70ºC
–40ºC to 85ºC
8-Pin SOIC (S08)
8-Pin SOIC (S08)
8-Pin SOIC (S08)
© Micro Linear 1997. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners.
Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502;
5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167. Japan: 2,598,946;
2,619,299; 2,704,176. Other patents are pending.
Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability
arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits
contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits
infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult
with appropriate legal counsel before deciding on a particular application.
10
2092 Concourse Drive
San Jose, CA 95131
Tel: 408/433-5200
Fax: 408/432-0295
www.microlinear.com
DS4863-01
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