MAX618EEE [MAXIM]
28V, PWM, Step-Up DC-DC Converter; 28V ， PWM ，升压型DC- DC转换器
| 型号： | MAX618EEE |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 28V, PWM, Step-Up DC-DC Converter |
| 文件： | 总14页 (文件大小：205K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1462; Rev 0; 6/99
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
Ge n e ra l De s c rip t io n
Fe a t u re s
The MAX618 CMOS, PWM, step-up DC-DC converter
g e ne ra te s outp ut volta g e s up to 28V a nd a c c e p ts
inputs from +3V to +28V. An internal 2A, 0.3Ω switch
eliminates the need for external power MOSFETs while
supplying output currents up to 500mA or more. A
PWM control scheme combined with Idle Mode™ oper-
ation at light loads minimizes noise and ripple while
maximizing efficiency over a wide load range. No-load
operating current is 500µA, which allows efficiency up
to 93%.
♦ Adjustable Output Voltage Up to +28V
♦ Up to 93% Efficiency
♦ Wide Input Voltage Range (+3V to +28V)
♦ Up to 500mA Output Current at +12V
♦ 500µA Quiescent Supply Current
♦ 3µA Shutdown Current
♦ 250kHz Switching Frequency
♦ Small 1W 16-Pin QSOP Package
A fast 250kHz switching frequency allows the use of
small surface-mount inductors and capacitors. A shut-
down mode extends battery life when the device is not
in us e . Ad a p tive s lop e c omp e ns a tion a llows the
MAX618 to accommodate a wide range of input and
output voltages with a simple, single compensation
capacitor.
The MAX618 is available in a thermally enhanced 16-
pin QSOP package that is the same size as an industry-
s ta nd a rd 8-p in SO b ut d is s ip a te s up to 1W. An
e va lua tion kit (MAX618EVKIT) is a va ila b le to he lp
speed designs.
Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
PIN-PACKAGE
MAX618EEE
-40°C to +85°C
16 QSOP
Ap p lic a t io n s
Automotive-Powered DC-DC Converters
Industrial +24V and +28V Systems
LCD Displays
Typ ic a l Op e ra t in g Circ u it
Palmtop Computers
P in Co n fig u ra t io n
V
IN
3V TO 28V
V
OUT
IN
LX
UP TO 28V
TOP VIEW
MAX618
GND
LX
1
2
3
4
5
6
7
8
16 GND
15 PGND
14 PGND
13 PGND
12 GND
11 VL
SHDN
PGND
LX
LX
MAX618
VL
FB
SHDN
COMP
FB
10 IN
COMP
GND
GND
9 GND
QSOP
Idle Mode is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
ABSOLUTE MAXIMUM RATINGS
IN to GND...............................................................-0.3V to +30V
LX to GND ..............................................................-0.3V to +30V
VL to GND ................................................................-0.3V to +6V
SHDN, COMP, FB to GND............................-0.3V to (VL + 0.3V)
PGND to GND.....................................................................±0.3V
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10sec) .............................+300°C
Continuous Power Dissipation (T = +70°C) (Note 1)
A
16-Pin QSOP (derate 15mW/°C above +70°C)...................1W
MAX618
2
Note 1: With part mounted on 0.9 in. of copper.
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V = +6V, PGND = GND, C = 4.7µF, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
IN
VL
A
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
28
UNITS
V
Input Voltage
V
IN
3
Supply Current, No Load
I
500
5
700
µA
V
= 3V to 28V, V = 1.6V, SHDN = VL
IN
IN
FB
Supply Current, Full Load, VL
Connected to IN
I
IN
6.5
3.5
mA
mA
V
IN
= 3V to 5.5V, V = 1.4V, SHDN = VL = IN
FB
V
= 3.4V to 28V, V = 1.4V, SHDN = VL,
FB
IN
Supply Current, Full Load
I
IN
2.5
V
VL
< V
IN
Shutdown Supply Current
VL Output Voltage
VL Load Regulation
VL Undervoltage Lockout
FB Set Voltage
I
3
8
µA
V
V
= 28V, V = 1.6V, SHDN = GND
IN
IN
FB
V
V
IN
= 3.5V or 28V, no load
2.9
3.05
25
3.2
40
VL
∆V
I
= 0 to 2mA, V = 1.6V
mV
V
VL
LOAD
FB
Rising edge, 1% hysteresis
2.58
1.47
2.7
1.5
1
2.8
1.53
50
V
V
FB
FB Input Bias Current
Line Regulation
I
FB
V
FB
= 1.6V
nA
%/V
%
∆V
V
IN
= 3V to 6V, V = 12V
OUT
0.01
0.2
0.08
OUT
Load Regulation
∆V
V
OUT
= 12V, I = 10mA to 500mA
LOAD
OUT
LX Voltage
V
LX
28
V
LX Switch Current Limit
I
PWM mode
1.7
2.2
2.7
A
LXON
Idle Mode Current-Limit
Threshold
0.25
0.35
0.45
A
LX On-Resistance
R
0.3
0.02
200
0.6
10
Ω
LXON
LX Leakage Current
I
V
= 28V
µA
µA
LXOFF
LX
COMP Maximum Output Current
I
FB = GND
100
0.8
COMP
COMP Current vs. FB Voltage
Transconductance
∆FB = 0.1V
1
mmho
V
IL
0.8
V
V
SHDN Input Logic Low
SHDN Input Logic High
Shutdown Input Current
Switching Frequency
Maximum Duty Cycle
V
IH
2.0
1
µA
kHz
%
SHDN = GND or VL
f
200
90
250
95
300
DC
2
_______________________________________________________________________________________
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
ELECTRICAL CHARACTERISTICS
(V = +6V, PGND = GND, C = 4.7µF, T = -40°C to +85°C, unless otherwise noted.) (Note 2)
IN
VL
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
28
UNITS
V
Input Voltage
V
IN
3
Supply Current, No Load
I
IN
800
µA
V
IN
= 3V to 28V, V = 1.6V, SHDN = VL
FB
Supply Current, Full Load,
VL Connected to IN
I
7.5
4
mA
mA
V
= 3V to 5.5, V = 1.4V, SHDN = VL = IN
IN
IN
FB
V
= 3.4V to 28V, V = 1.4V, SHDN = VL,
FB
IN
Supply Current, Full Load
I
IN
VL < V
IN
Supply Current Shutdown
VL Output Voltage
I
10
3.3
µA
V
V
= 28V, V = 1.6V, SHDN = GND
IN
IN
FB
V
V
IN
= 3.5V or 28V, no load
2.85
2.55
VL
VL Undervoltage Lockout
FB Set Voltage
V
Rising edge, 1% hysteresis
2.85
1.545
28
V
VL
V
1.455
V
FB
LX Voltage Range
V
V
LXON
LX Switch Current Limit
LX On-Resistance
I
PWM mode
1.4
3
A
LXON
R
0.6
Ω
LXON
f
Switching Frequency
188
312
kHz
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(Circuit of Figure 1, T = +25°C.)
A
EFFICIENCY vs. OUTPUT CURRENT
(V = 12V)
EFFICIENCY vs. OUTPUT CURRENT
(V = 28V)
OUT
OUT
100
100
V
IN
= 12V
V
= 8V
IN
90
80
70
60
50
40
30
20
10
90
80
70
60
50
40
30
20
10
V
= 5V
IN
V
IN
= 3V
V
IN
= 5V
V
= 3V
IN
0
0
0.1
1
10
100
1000
0.1
1
10
100
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
_______________________________________________________________________________________
3
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 1, T = +25°C.)
A
NO-LOAD SUPPLY CURRENT
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE
vs. INPUT VOLTAGE
SUPPLY CURRENT vs. TEMPERATURE
700
650
600
550
500
450
400
350
300
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.65
V
= 3V
IN
MAX618
0.60
0.55
0.50
0.45
0.40
V
IN
= 5V
V
= 8V
IN
INCLUDES CAPACITOR LEAKAGE CURRENT
2
7
12
17
22
27
32
0
5
10
15
20
25
30
-50 -30 -10 10 30 50 70 90 110
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
INPUT VOLTAGE (V)
HEAVY-LOAD SWITCHING
MEDIUM-LOAD SWITCHING
WAVEFORMS
WAVEFORMS
LINE-TRANSIENT RESPONSE
MAX618 toc08
MAX618 toc07
MAX618 toc09
I
L
V
OUT
(1A/div)
(50mV/div)
I
L
(1A/div)
0
0
V
LX
V
LX
(10V/div)
(10V/div)
V
V
OUT
6V
3V
OUT
V
(5V/div)
IN
(100mV/div)
(100mV/
div)
2µs/div
= 5V, V = 12V, I = 500mA
2µs/div
2ms/div
V
IN
OUT
OUT
I
= 200mA, V = 12V
V
IN
= 5V, V = 12V, I = 200mA
OUT OUT
OUT
OUT
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
SHUTDOWN RESPONSE
LOAD-TRANSIENT RESPONSE
MAX618 toc11
MAX618 toc10
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
V
= 12V
OUT
V
SHDN
(2V/div)
OUT
(200mV/div)
0
12V
5V
I
OUT
(100mA/div)
V
OUT
0
(2V/div)
500µs/div
2
3
4
5
6
7
8
9
10 11 12
5ms/div
INPUT VOLTAGE (V)
V
IN
= 5V, V = 12V, I
= 500mA
OUT
LOAD
V
IN
= 5V, V = 12V
OUT
4
_______________________________________________________________________________________
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
P in De s c rip t io n
PIN
NAME
GND
LX
FUNCTION
1, 8, 9,
12, 16
Ground
2, 3, 4
5
Drain of internal N-channel switch. Connect the inductor between IN and LX.
Shutdown Input. A logic low puts the MAX618 in shutdown mode and reduces supply current to 3µA.
SHDN must not exceed VL. In shutdown, the output falls to V less one diode drop.
SHDN
IN
6
7
COMP
FB
Compensation Input. Bypass to GND with the capacitance value shown in Table 2.
Feedback Input. Connect a resistor-divider network to set V
. FB threshold is 1.5V.
OUT
LDO Regulator Supply Input. IN accepts inputs up to +28V. Bypass to GND with a 1µF ceramic capacitor
as close to pins 10 and 12 as possible.
10
IN
11
VL
Internal 3.1V LDO Regulator Output. Bypass to GND with a 4.7µF capacitor.
Power Ground, source of internal N-channel switch
13, 14, 15
PGND
_______________ De t a ile d De s c rip t io n
The MAX618 pulse-width modulation (PWM) DC-DC
L
3V TO 28V
V
converter with an internal 28V switch operates in a wide
ra ng e of DC-DC c onve rs ion a p p lic a tions inc lud ing
boost, SEPIC, and flyback configurations. The MAX618
IN
C
IND
uses fixed-frequency PWM operation and Maxim’s pro-
ECB1Q503L
prietary Idle Mode control to optimize efficiency over a
wide range of loads. It also features a shutdown mode
to minimize quiescent current when not in operation.
V
OUT
LX
IN
UP TO 28V
1µF
C
OUT
MAX618
R1
SHDN
PGND
P WM Co n t ro l S c h e m e a n d
Id le Mo d e Op e ra t io n
The MAX618 combines continuous-conduction PWM
operation at medium to high loads and Idle Mode oper-
ation at light loads to provide high efficiency over a
wide range of load conditions. The MAX618 control
scheme actively monitors the output current and auto-
matically switches between PWM and Idle Mode to
optimize efficiency and load regulation. Figure 2 shows
a functional diagram of the MAX618’s control scheme.
V
FB
L
4.7µF
C
P
R2
GND
COMP
C
COMP
The MAX618 normally operates in low-noise, continu-
ous-conduction PWM mode, switching at 250kHz. In
PWM mode, the internal MOSFET switch turns on with
each clock pulse. It remains on until either the error
comparator trips or the inductor current reaches the 2A
switch-current limit. The error comparator compares the
feedback-error signal, current-sense signal, and slope-
compensation signal in one circuit block. When the
switch turns off, energy transfers from the inductor to
V
R1
R2
C
L
C
C
C
COMP
OUT
IND
OUT
P
8V
402kΩ 93.1kΩ 150µF
100µF
12µH
15µH
39µH
150µF 220pF 0.082µF
100µH 56pF
33µF 47pF
12V 715kΩ 100kΩ
28V 574kΩ 32.4kΩ 86µF
0.1µF
0.47µF
Figure 1. Single-Supply Operation
_______________________________________________________________________________________________________
5
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
IDLE MODE
CURRENT LIMIT
MAX618
PWM
CURRENT LIMIT
CURRENT-
SENSE
IN
MAX618
PGND
CIRCUIT
VL
ERROR
COMPARATOR
PWM
LOGIC
NMOS
R
LX
FB
OUT
250kHz
OSCILLATOR
14R
GND
SLOPE
COMPENSATION
REFERENCE
INTEGRATOR
COMP
LINEAR
REGULATOR
THERMAL
SHUTDOWN
SHDN
SHUTDOWN
IN
VL
Figure 2. Functional Diagram
the output capacitor. Output current is limited by the 2A
MOSFET c urre nt limit a nd the MAX618’s p a c ka g e
p owe r-d is s ip a tion limit. Se e the Ma ximum Outp ut
Current section for details.
ranges. The MAX618 uses both control schemes in par-
allel: the dominant, low-frequency components of the
error signal are tightly regulated with a voltage-control
loop, while a current-control loop improves stability at
higher frequencies. Compensation is achieved through
In Idle Mode, the MAX618 improves light-load efficien-
cy by reducing inductor current and skipping cycles to
reduce the losses in the internal switch, diode, and
inductor. In this mode, a switching cycle initiates only
when the error comparator senses that the output volt-
a g e is a b out to d rop out of re g ula tion. Whe n this
occurs, the NMOS switch turns on and remains on until
the inductor current exceeds the nominal 350mA Idle
Mode current limit.
the selection of the output capacitor (C
), the inte-
OUT
grator capacitor (C
), and the pole capacitor (C )
COMP
P
from FB to GND. C cancels the zero formed by C
P
OUT
and its ESR. Refer to the Capacitor Selection section for
guidance on selecting these capacitors.
VL Lo w -Dro p o u t Re g u la t o r
The MAX618 contains a 3.1V low-dropout linear regula-
tor to power internal circuitry. The regulator’s input is IN
and its output is VL. The IN to VL dropout voltage is
100mV, so that when IN is less than 3.2V, VL is typically
100mV below IN. The MAX618 still operates when the
LDO is in dropout, as long as VL remains above the
2.7V undervoltage lockout. Bypass VL with a 4.7µF
ceramic capacitor placed as close to the VL and GND
pins as possible.
Refer to Table 1 for an estimate of load currents at which
the MAX618 transitions between PWM and Idle Mode.
Co m p e n s a t io n S c h e m e
Although the higher loop gain of voltage-controlled
architectures tends to provide tighter load regulation,
current-controlled architectures are generally easier to
c omp e ns a te ove r wid e inp ut a nd outp ut volta g e
6
_______________________________________________________________________________________
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
_______________________________________________________________________________________
7
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
VL can be overdriven by an external supply between
2.7V and 5.5V. In systems with +3.3V or +5V logic
power supplies available, improve efficiency by power-
The circuit in Figure 3 allows a logic supply to power
the MAX618 while using a separate source for DC-DC
conversion power (inductor voltage). The logic supply
(between 2.7V and 5.5V) connects to VL and IN. VL =
IN; voltages of 3.3V or more improve efficiency by pro-
viding greater gate drive for the internal MOSFET.
ing VL and V directly from the logic supply as shown
IN
in Figure 3.
Op e ra t in g Co n fig u ra t io n s
The MAX618 can be connected in one of three configura-
tions described in Table 2 and shown in Figures 1, 3, and
4. The VL linear regulator allows operation from a single
supply between +3V and +28V as shown in Figure 1.
The c irc uit in Fig ure 4 a llows s e p a ra te s up p lie s to
power IN and the inductor voltage. It differs from the
connection in Figure 3 in that the MAX618 chip supply
is not limited to 5.5V.
MAX618
Table 2. Input Configurations
V
INDUCTOR
VOLTAGE
IN
CIRCUIT
CONNECTION
BENEFITS/COMMENTS
RANGE
•
Single-supply operation.
Input voltage connects
to IN and inductor.
3V to V
(up to 28V)
•
SHDN must be connected to or pulled up to VL. On/off
control requires an open-drain or open-collector connection
to SHDN.
OUT
Figure 1
V
IN
•
•
Increased efficiency.
SHDN can be driven by logic powered from the supply con-
nected to IN and VL, or can be connected to or pulled up to
VL.
IN and VL connect
together. Inductor volt-
age supplied by a
separate source.
0 to V
(up to 28V)
OUT
Figure 3
2.7V to 5.5V
•
Input power source (inductor voltage) is separate from the
MAX618’s bias (V = VL) and can be less than or greater
IN
than V .
IN
•
•
Input power source (inductor voltage) is separate from the
MAX618’s bias (V ) and can be less than or greater than
IN
IN and inductor volt-
age supplied by sepa-
rate sources.
V
IN
.
0 to V
(up to 28V)
OUT
Figure 4
3V to 28V
SHDN must be connected to or pulled up to VL. On/off
control requires an open-drain or open-collector connection
to SHDN.
V
V
IND
IND
L
L
UP TO 28V
UP TO 28V
C
IND
C
IND
OUT
UP TO 28V
IN
IN
OUT
UP TO 28V
3V TO 28V
2.7V TO 5.5V
IN
IN
LX
LX
1µF
1µF
C
OUT
C
OUT
MAX618
MAX618
R1
SHDN
PGND
SHDN
R1
R2
PGND
FB
VL
VL
4.7µF
FB
4.7µF
COMP
C
P
COMP
C
P
R2
C
COMP
C
COMP
GND
GND
Figure 3. Dual-Supply Operation (V = 2.7V to 5.5V)
Figure 4. Dual-Supply Operation (V = 3V to 28V)
IN
IN
8
_______________________________________________________________________________________
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
MAX618
MAX618
VL
IN
SYSTEM
LOGIC SUPPLY
OPEN-DRAIN
LOGIC
100k
VL
SHDN
ON/OFF
CONTROL
SHDN
SYSTEM LOGIC
ON/OFF
CONTROL
Figure 5. Adding On/Off Control to Circuit of Figure 1 or 4
Figure 6. Adding On/Off Control to Circuit of Figure 3
S h u t d o w n Mo d e
De t e rm in in g t h e In d u c t o r Va lu e
The MAX618’s high switching frequency allows the use
of a small value inductor. The recommended inductor
value is proportional to the output voltage and is given
by the following:
In shutdown mode (SHDN = 0), the MAX618’s feed-
back and control circuit, reference, and internal biasing
circuitry turn off and reduce the IN supply current to
3µA (10µA max). When in shutdown, a current path
remains from the input to the output through the exter-
nal inductor and diode. Consequently, the output falls
V
OUT
L =
5
to V less one diode drop in shutdown.
7 10
IN
SHDN may not exceed VL. For always-on operation,
connect SHDN to VL. To add on/off control to the circuit
of Figure 1 or 4, pull SHDN to VL with a resistor (10kΩ
to 100kΩ) and drive SHDN with an open-drain logic
gate or switch as shown in Figure 5. Alternatively, the
circuit of Figure 3 allows direct SHDN drive by any
logic-level gate powered from the same supply that
powers VL and IN, as shown in Figure 6.
After solving for the above equation, round down as
necessary to select a standard inductor value.
Whe n s e le c ting a n ind uc tor, c hoos e one ra te d to
250kHz, with a saturation current exceeding the peak
ind uc tor c urre nt, a nd with a DC re s is ta nc e und e r
200mΩ. Ferrite core or equivalent inductors are gener-
a lly a p p rop ria te (s e e MAX618 EV kit d a ta s he e t).
Calculate the peak inductor current with the following
equation:
__________________De s ig n P ro c e d u re
V
− V
IN
The MAX618 operates in a number of DC-DC converter
configurations including step-up, SEPIC, and flyback.
The following design discussion is limited to step-up
converters.
OUT
(
)
V
V
IN
OUT
I
= I
+ 2µs
LX(PEAK)
OUT
V
L
V
OUT
IN
Note that the peak inductor current is internally limited
to 2A.
S e t t in g t h e Ou t p u t Vo lt a g e
Two external resistors (R1 and R2) set the output volt-
age. First, select a value for R2 between 10kΩ and
200kΩ. Calculate R1 with:
Dio d e S e le c t io n
The MAX618’s high switching frequency demands a
high-speed rectifier. Schottky diodes are preferred for
most applications because of their fast recovery time
and low forward voltage. Make sure that the diode’s
peak current rating exceeds the 2A peak switch cur-
rent, and that its breakdown voltage exceeds the out-
put voltage.
V
OUT
R1=R2
where V is 1.5V.
−1
V
FB
FB
_______________________________________________________________________________________
9
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
needed for the output capacitances specified in Table
Ma x im u m Ou t p u t Cu rre n t
The MAX618’s 2.2A LX current limit determines the
output power that can be supplied for most applica-
tions. In some cases, particularly when the input volt-
age is low, output power is sometimes restricted by
package dissipation limits. The MAX618 is protected
by a thermal shutdown circuit that turns off the switch
when the die temperature exceeds +150°C. When the
device cools by 10°C, the switch is enabled again.
Table 3 details output current with a variety of input and
output voltages. Each listing in Table 3 is either the limit
set by an LX current limit or by package dissipation at
+85°C ambient, whichever is lower. The values in Table
3 assume a 40mΩ inductor resistance.
4. However, if a different output capacitor is used (e.g.,
a standard value), then recalculate the value of capaci-
tance needed for the integrator capacitor with the fol-
lowing formula:
C
(Table 5) C
OUT
COMP
C
=
COMP
MAX618
C
(Table 4)
OUT
Pole Compensation Capacitor
The pole capacitor (C ) cancels the unwanted zero
P
introduced by C
’s ESR, and thereby ensures stabil-
OUT
ity in PWM op e ra tion. The e xa c t va lue of the p ole
capacitor is not critical, but it should be near the value
calculated by the following equation:
Ca p a c it o r S e le c t io n
Input Capacitors
, reduces the input
ripple created by the boost configuration. High-imped-
ance sources require high C values. However, 68µF
is generally adequate for input currents up to 2A. Low
ESR capacitors are recommended because they will
decrease the ripple created on the input and improve
efficiency. Capacitors with ESR below 0.3Ω are gener-
ally appropriate.
R
C
(R2 + R2)
OUT
ESR
C
=
The input bypass capacitor, C
IND
P
R1 R2
IND
where R
is C
’s ESR.
OUT
ESR
La yo u t Co n s id e ra t io n s
Proper PC board layout is essential due to high current
levels and fast switching waveforms that radiate noise.
Use the MAX618 evaluation kit or equivalent PC layout
to perform initial prototyping. Breadboards, wire-wrap,
and proto-boards are not recommended when proto-
typing switching regulators.
In addition to the input bypass capacitor, bypass IN
with a 1µF ceramic capacitor placed as close to the IN
and GND pins as possible. Bypass VL with a 4.7µF
ceramic capacitor placed as close to the VL and GND
pins as possible.
It is imp orta nt to c onne c t the GND p in, the inp ut
bypass capacitor ground lead, and the output filter
capacitor ground lead to a single point to minimize
ground noise and improve regulation. Also, minimize
lead lengths to reduce stray capacitance, trace resis-
tance, and radiated noise, with preference given to the
feedback circuit, the ground circuit, and LX. Place the
feedback resistors as close to the FB pin as possible.
Place a 1µF input bypass capacitor as close as possi-
ble to IN and GND.
Output Capacitor
Use Table 4 to find the minimum output capacitance
ne c e s s a ry to e ns ure s ta b le op e ra tion. In a d d ition,
choose an output capacitor with low ESR to reduce the
output ripple. The dominant component of output ripple
is the product of the peak-to-peak inductor ripple cur-
rent and the ESR of the output capacitor. ESR below
50mΩ generates acceptable levels of output ripple for
most applications.
Refer to the MAX618 evaluation kit for an example of
proper board layout.
Integrator Capacitor
The compensation capacitor (C
) sets the domi-
COMP
nant pole in the MAX618’s transfer function. The proper
c omp e ns a tion c a p a c ita nc e d e p e nd s up on outp ut
capacitance. Table 5 shows the capacitance value
10 ______________________________________________________________________________________
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
______________________________________________________________________________________ 11
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
12 ______________________________________________________________________________________
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
MAX618
______________________________________________________________________________________ 13
2 8 V, P WM, S t e p -Up DC-DC Co n ve rt e r
P a c k a g e In fo rm a t io n
MAX618
___________________Ch ip In fo rm a t io n
TRANSISTOR COUNT: 1794
14 ______________________________________________________________________________________
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