MAXIM MAX608ESA

19-0438; Rev 0; 9/95
NUAL
KIT MA
ATION
EET
H
S
A
EVALU
T
WS DA
FOLLO
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
________________________Applications
High-Efficiency DC-DC Converters
____________________________Features
♦ 1.8V to 16.5V Input Range
♦ 85% Efficiency for 30mA to 1.5A Load Currents
♦ Up to 10W Output Power
♦ 110µA Max Supply Current
♦ 5µA Max Shutdown Current
♦ Preset 5V or Adjustable Output (3V to 16.5V)
♦ Current-Limited PFM Control Scheme
♦ Up to 300kHz Switching Frequency
♦ Evaluation Kit Available
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
Battery-Powered Applications
MAX608C/D
0°C to +70°C
Positive LCD-Bias Generators
MAX608EPA
-40°C to +85°C
8 Plastic DIP
Portable Communicators
MAX608ESA
-40°C to +85°C
8 SO
Dice*
* Contact factory for dice specifications.
__________Typical Operating Circuit
INPUT
1.8V TO VOUT
ON/OFF
__________________Pin Configuration
TOP VIEW
OUTPUT
5V
MAX608
SHDN
REF
EXT
CS
FB AGND GND OUT
N
EXT
1
8
CS
OUT
2
7
GND
FB 3
6
AGND
SHDN 4
5
REF
MAX608
DIP/SO
________________________________________________________________ Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
1
MAX608
_______________General Description
The MAX608 low-voltage step-up controller operates
from a 1.8V to 16.5V input voltage range. Pulse-frequency-modulation (PFM) control provides high efficiency at heavy loads, while using only 85µA (typical)
when operating with no load. In addition, a logic-controlled shutdown mode reduces supply current to 2µA
typical. The output voltage is factory-set at 5V or can be
adjusted from 3V to 16.5V with an external resistor
divider.
The MAX608 is ideal for two- and three-cell batterypowered systems. An operating frequency of up to
300kHz allows use with small surface-mount components.
The MAX608 operates in “bootstrapped” mode only
(with the chip supply, OUT, connected to the DC-DC
output). For a 12V output without external resistors, or
for nonbootstrapped applications (chip supply connected to input voltage), refer to the pin-compatible
MAX1771. The MAX608 is available in 8-pin DIP and
SO packages.
MAX608
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
ABSOLUTE MAXIMUM RATINGS
Supply Voltage
OUT to GND.............................................................-0.3V, 17V
EXT, CS, REF, SHDN, FB to GND ...............-0.3V, (VOUT + 0.3V)
GND to AGND.............................................................0.1V, -0.1V
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW
SO (derate 5.88mW/°C above +70°C) .........................471mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
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
(VOUT = 5V, ILOAD = 0mA, TA = -40°C to +85°C where indicated. TA = -25°C to +85°C for all other limits. Typical values are at
TA = +25°C.)
PARAMETER
SYMBOL
MIN
TYP
MAX
TA = -25°C to +85°C
1.8
16.5
TA = -40°C to +85°C (Note 1)
1.9
16.5
Minimum Start-Up Voltage
No load
Supply Current
Output Voltage (Note 3)
UNITS
V
1.6
1.8
V
85
110
µA
VOUT = 16.5V,
SHDN ≤ 0.4V
TA = -25°C to +85°C
VOUT = 10V,
SHDN ≥ 1.6V
TA = -25°C to +85°C
VIN = 2.0V to 5.0V,
over full load range,
circuit of Figure 2a
TA = -25°C to +85°C
4.825
5.0
5.175
TA = -40°C to +85°C (Note 1)
4.800
5.0
5.200
TA = -40°C to +85°C (Note 1)
120
2
TA = -40°C to +85°C (Note 1)
5
10
µA
V
Output Voltage Line
Regulation (Note 4)
VIN = 2.7V to 4.0V, VOUT = 5V, ILOAD = 500mA,
circuit of Figure 2a
7
mV/V
Output Voltage Load
Regulation (Note 4)
VIN = 2V, VOUT = 5V, ILOAD = 0mA to 500mA,
circuit of Figure 2a
60
mV/A
Maximum Switch On-Time tON(max)
12
16
20
µs
Minimum Switch Off-Time
1.8
2.3
2.8
µs
tOFF(min)
VIN = 4V, VOUT = 5V, ILOAD = 500mA,
circuit of Figure 2a
Efficiency
Reference Voltage
2
CONDITIONS
Input Voltage Range
(Note 2)
VREF
IREF = 0µA
87
TA = -25°C to +85°C
1.4625
TA = -40°C to +85°C (Note 1)
1.4475
1.5
%
1.5375
1.5525
V
REF Load Regulation
0µA ≤ IREF ≤ 100µA
-4
10
mV
REF Line Regulation
3V ≤ VOUT ≤ 16.5V
40
100
µV/V
TA = -25°C to +85°C
1.4625
TA = -40°C to +85°C (Note 1)
1.4475
1.5
1.5375
FB Trip Point Voltage
(Note 5)
VFB
FB Input Current
IFB
SHDN Input High Voltage
VIH
VOUT = 1.8V to 16.5V
SHDN Input Low Voltage
VIL
VOUT = 1.8V to 16.5V
0.4
SHDN Input Current
IIN
VOUT = 16.5V, SHDN = 0V or 16.5V
±1
1.5525
-4
TA = -25°C to +85°C
TA = -40°C to +85°C (Note 1)
±20
±40
1.6
_______________________________________________________________________________________
V
nA
V
V
µA
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
(VOUT = 5V, ILOAD = 0mA, TA = -40°C to +85°C where indicated. TA = -25°C to +85°C for all other limits. Typical values are at
TA = +25°C.)
PARAMETER
SYMBOL
Current-Limit Trip Level
VCS
CS Input Current
ICS
CONDITIONS
VOUT = 3V to 16.5V
MIN
TYP
MAX
TA = -25°C to +85°C
85
100
115
TA = -40°C to +85°C (Note 1)
80
UNITS
mV
120
0.01
EXT Rise Time
VOUT = 5V, 1nF from EXT to GND
50
EXT Fall Time
VOUT = 5V, 1nF from EXT to GND
50
EXT On-Resistance
EXT = high or low
15
µA
±1
ns
Ω
30
Note 1: Limits over this temperature range are guaranteed by design.
Note 2: The MAX608 must be operated in bootstrapped mode with OUT connected to the DC-DC circuit output. The minimum output
voltage set point is +3V.
Note 3: Output voltage guaranteed using preset voltages. See Figures 4a–4d for output current capability versus input voltage.
Note 4: Output voltage line and load regulation depend on external circuit components.
Note 5: Operation in the external-feedback mode is guaranteed to be accurate to the VFB trip level, and does not include resistor tolerance.
__________________________________________Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(VOUT = 12V)
VIN = 9.0V
VIN = 3.0V
100
VIN = 2.0V
VIN = 2.0V
VIN = 3.0V
70
80
60
1
1000
10
1
10
400
400
300
200
100
1000
LOAD CURRENT (mA)
SUPPLY CURRENT
vs. INPUT VOLTAGE
VOUT = 12V
CIRCUIT OF FIGURE 2b
EXTERNAL FET THRESHOLD LIMITS
FULL-LOAD START-UP BELOW 3.6V
200
300
200
MAX608-06
500
LOAD CURRENT (mA)
VOUT = 5V
CIRCUIT OF FIGURE 2a
EXTERNAL FET THRESHOLD LIMITS
FULL-LOAD START-UP BELOW 3.7V
MAX608-04
700
500
1000
LOAD CURRENT vs.
MINIMUM START-UP
INPUT VOLTAGE
LOAD CURRENT vs.
MINIMUM START-UP
INPUT VOLTAGE
600
100
LOAD CURRENT (mA)
SUPPLY CURRENT (µA)
100
LOAD CURRENT (mA)
MAX608-05
10
VIN = 2.0V
70
60
1
VIN = 3.0V
90
80
60
LOAD CURRENT (mA)
VIN = 5.0V
EFFICIENCY (%)
80
70
VIN = 6.0V
90
VIN = 3.5V
EFFICIENCY (%)
EFFICIENCY (%)
90
100
MAX608-03
VIN = 4.0V
MAX608-01
100
EFFICIENCY vs. LOAD CURRENT
(VOUT = 3.3V)
MAX608-02
EFFICIENCY vs. LOAD CURRENT
(VOUT = 5V)
150
100
50
100
100
0
0
0
1.8
2.2
2.6
3.0
3.4
3.8
MINIMUM START-UP VOLTAGE (V)
4.0
1.8
2.2
2.6
3.0
3.4
3.8
MINIMUM START-UP VOLTAGE (V)
4.0
0
1
2
3
4
5
INPUT VOLTAGE (V)
_______________________________________________________________________________________
3
MAX608
ELECTRICAL CHARACTERISTICS (continued)
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
REFERENCE OUTPUT RESISTANCE vs.
TEMPERATURE
CEXT = 2200pF
CEXT = 1000pF
CEXT = 470pF
CEXT = 100pF
100
50
MAX608-08
10µA
1.502
150
100
50µA
6
4
1.494
12
10
8
1.492
-60 -40 -20
0 20 40 60 80 100 120 140
MAXIMUM SWITCH ON-TIME vs.
TEMPERATURE
SHUTDOWN CURRENT (µA)
3.0
2.5
2.0
V+ = 15V
1.5
V+ = 8V
V+ = 4V
2.20
0
90
120 150
2.25
1.0
0.5
60
2.30
MAX608-11
3.5
15.5
30
MINIMUM SWITCH OFF-TIME vs.
TEMPERATURE
SHUTDOWN CURRENT vs. TEMPERATURE
16.0
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
4.0
MAX608-10
16.5
0
-60 -40 -20
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
-60 -30
1.498
tOFF(min) (µs)
2
1.500
1.496
100µA
50
0
0
1.504
200
MAX608-12
150
REFERENCE vs. TEMPERATURE
1.506
REFERENCE (V)
MAX608-07
200
250
REFERENCE OUTPUT RESISTANCE (Ω)
EXT RISE/FALL TIME (ns)
250
MAX608-09
EXT RISE/FALL TIME vs. SUPPLY VOLTAGE
tON(max) (µs)
MAX608
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
-60 -40 -20
TEMPERATURE (°C)
0 20 40 60 80 100 120 140
-60 -30
0
30
VOUT
VOUT
A
B
90
MEDIUM-LOAD SWITCHING WAVEFORMS
(VOUT = 5V)
HEAVY-LOAD SWITCHING WAVEFORMS
(VOUT = 5V)
A
60
TEMPERATURE (°C)
TEMPERATURE (°C)
0V
0V
ILIM
ILIM
B
0A
0A
C
C
2µs/div
VIN = 3V, IOUT = 930mA, VOUT = 5V
A = EXT VOLTAGE, 5V/div
B = INDUCTOR CURRENT, 1A/div
C = VOUT RIPPLE, 50mV/div, AC-COUPLED
4
20µs/div
VIN = 3V, IOUT = 490mA, VOUT = 5V
A = EXT VOLTAGE, 5V/div
B = INDUCTOR CURRENT, 1A/div
C = VOUT RIPPLE, 50mV/div, AC-COUPLED
_______________________________________________________________________________________
120 150
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
MEDIUM-LOAD SWITCHING WAVEFORMS
(VOUT = 12V)
HEAVY-LOAD SWITCHING WAVEFORMS
(VOUT = 12V)
VOUT
VOUT
A
A
0V
0V
ILIM
ILIM
B
B
0A
0A
C
C
10µs/div
2µs/div
VIN = 4V, IOUT = 300mA, VOUT = 12V
A = EXT VOLTAGE, 10V/div
B = INDUCTOR CURRENT, 1A/div
C = VOUT RIPPLE, 50mV/div, AC-COUPLED
VIN = 4V, IOUT = 490mA, VOUT = 12V
A = EXT VOLTAGE, 10V/div
B = INDUCTOR CURRENT, 1A/div
C = VOUT RIPPLE, 50mV/div, AC-COUPLED
LINE-TRANSIENT RESPONSE
(VOUT = 5V)
LOAD-TRANSIENT RESPONSE
(VOUT = 5V)
A
500mA
4.0V
A
0A
2.7V
B
B
2ms/div
5ms/div
IOUT = 500mA, VOUT = 5V
A = VIN, 2.7V TO 4.0V, 1V/div
B = VOUT RIPPLE, 100mV/div, AC-COUPLED
VIN = 2V, VOUT = 5V
A = LOAD CURRENT, 0mA TO 500mA, 500mA/div
B = VOUT RIPPLE, 50mV/div, AC-COUPLED
EXITING SHUTDOWN
A
0V
5V
B
0V
200µs/div
IOUT = 500mA, VIN = 3.5V
A = SHDN, 2V/div
B = VOUT, 2V/div
_______________________________________________________________________________________
5
MAX608
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
MAX608
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
______________________________________________________________Pin Description
PIN
NAME
FUNCTION
1
EXT
Gate Drive for External N-Channel Power Transistor
2
OUT
Power-Supply and Voltage-Sense Input. Always connect OUT to circuit output.
3
FB
4
SHDN
5
REF
6
AGND
7
GND
8
CS
Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation.
Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section.
Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, VOUT is a diode drop
below the input voltage (due to the DC path from the input voltage to the output). Connect to
ground for normal operation.
1.5V Reference Output that can source 100µA for external loads. Bypass to GND with 0.1µF.
The reference is disabled in shutdown.
Analog Ground
High-Current Ground Return for the Output Driver
Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS
and AGND.
REF
FB
DUAL-MODE
COMPARATOR
SHDN
MAX608
BIAS
CIRCUITRY
50mV
1.5V
REFERENCE
ERROR
COMPARATOR
LOW-VOLTAGE
START-UP
COMPARATOR
MIN OFF-TIME
ONE-SHOT
Q
TRIG
N
OUT
2.3µs
F/F
S
Q
R
MAX ON-TIME
ONE-SHOT
LOW-VOLTAGE
OSCILLATOR
2.5V
EXT
TRIG
Q
16µs
CURRENT-SENSE
AMPLIFIER
0.1V
CS
Figure 1. Functional Diagram
6
_______________________________________________________________________________________
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
C2
0.1µF
5 REF
C3
0.1µF
4 SHDN
C2
0.1µF
2
2
OUT
OUT
MAX608
3 FB
EXT
1
5 REF
C1
150µF
L1
22µH
D1
1N5817
C3
0.1µF
VOUT = 5V
@ 0.5A
4 SHDN MAX608
6
N
AGND
L1
22µH
EXT
CS
1
CS
8
FB
RSENSE
50mΩ
GND
7
R2 = (R1)
D1
1N5817
VOUT = 12V
@ 0.3A
C4
200µF
8
RSENSE
50mΩ
3
GND
7
C4
200µF
C1
150µF
N
MMFT3055EL
MMFT3055EL
6 AGND
MAX608
VIN = 2V
VIN = 2V
R2
402k
R1
58k
( VVOUT -1)
REF
VREF = 1.5V
Figure 2a. 5V Preset Output
Figure 2b. 12V Output
_______________Detailed Description
The MAX608 is a BiCMOS, step-up, switch-mode power-supply controller that provides a preset 5V output, in
addition to adjustable-output operation. Its unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulse-width
modulation (high efficiency with heavy loads), providing
high efficiency over a wide output current range, as well
as increased output current capability over previous
PFM devices. In addition, the external sense resistor
and power transistor allow the user to tailor the output
current capability for each application. Figure 1 shows
the MAX608 functional diagram. The device has a shutdown mode that reduces the supply current to 5µA
max.
Figure 2 shows the standard application circuits. The
IC is powered from the output, and the input voltage
range is 1.8V to VOUT (this configuration is commonly
known as bootstrap operation). The voltage applied to
the gate of the external power transistor is switched
from VOUT to ground.
The MAX608’s output voltage can be set to 5V by connecting FB to ground; it can also be adjusted from 3V
to 16.5V using external resistors. Use 1% external feedback resistors when operating in adjustable-output
mode (Figures 2b, 2c) to achieve an overall output voltage accuracy of ±5%.
VIN = 2V
C2
0.1µF
2
5 REF
C3
0.1µF
4 SHDN MAX608
6
L1
22µH
OUT
AGND
EXT
CS
FB
GND
7
R2 = (R1)
( VVOUT -1)
1
C1
150µF
D1
1N5817
N
SI6426
VOUT = 3.3V
@ 0.6A
C4
200µF
8
3
RSENSE
50mΩ
R1
50k
R2
60k
C5
47pF
REF
VREF = 1.5V
Figure 2c. 3.3V Output
PFM Control Scheme
The MAX608 uses a proprietary current-limited PFM control scheme to provide high efficiency over a wide range
of load currents. This control scheme combines the ultralow supply current of PFM converters (or pulse skippers)
with the high full-load efficiency of PWM converters.
_______________________________________________________________________________________
7
MAX608
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
Unlike traditional PFM converters, the MAX608 uses a
sense resistor to control the peak inductor current. The
device also operates with high switching frequencies
(up to 300kHz), allowing the use of miniature external
components.
As with traditional PFM converters, the power transistor
is not turned on until the voltage comparator senses
the output is out of regulation. However, unlike traditional PFM converters, the MAX608 switch uses the combination of a peak current limit and a pair of one-shots
that set the maximum on-time (16µs) and minimum offtime (2.3µs); there is no oscillator. Once off, the minimum off-time one-shot holds the switch off for 2.3µs.
After this minimum time, the switch either 1) stays off if
the output is in regulation, or 2) turns on again if the
output is out of regulation.
The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high
efficiency with heavy loads. When the power switch is
turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16µs later), or 2) the
switch current reaches the peak current limit set by the
current-sense resistor.
The MAX608 switching frequency is variable (depending on load current and input voltage), causing variable
switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the
filter capacitor equivalent series resistance (ESR). For
example, when generating a 5V output at 500mA from
a 2V input, only 75mV of output ripple occurs, using the
circuit of Figure 2a.
Low-Voltage Start-Up Oscillator
The MAX608 features a low input voltage start-up oscillator that guarantees start-up with no load for input voltages down to 1.8V. At these low voltages, the output
voltage is not large enough for proper error-comparator
operation and internal biasing. The start-up oscillator
has a fixed 50% duty cycle and the MAX608 disregards
the error-comparator output when the output voltage is
less than 2.5V. Above 2.5V, the error-comparator and
normal one-shot timing circuitry are used.
R2
VOUT
FB
MAX608
R1
C5*
R1 = 10k TO 500k
GND
R2 = R1
V
-1)
( VOUT
REF
VREF = 1.5V
* OPTIONAL, SEE TEXT FOR VALUE
Figure 3. Adjustable Output Circuit
__________________Design Procedure
Setting the Output Voltage
The MAX608’s output voltage is preset to 5V (FB = 0V),
or it can be adjusted from 16.5V down to 3V using external resistors R1 and R2, configured as shown in Figure 3.
For adjustable-output operation, select feedback resistor
R1 in the 10kΩ to 500kΩ range. R2 is given by:
VOUT -1
R2 = (R1) –––––
VREF
(
)
where VREF equals 1.5V.
OUT must always be connected to the circuit output.
Figure 2 shows various circuit configurations for preset/
adjustable operation.
Determining RSENSE
Shutdown Mode
Use the theoretical output current curves shown in
Figures 4a–4d to select R SENSE . They are derived
using the minimum (worst-case) current-limit comparator threshold value over the extended temperature
range (-40°C to +85°C). No tolerance was included for
RSENSE. The voltage drop across the diode is assumed
to be 0.5V, and the drop across the power switch
rDS(ON) and coil resistance is assumed to be 0.3V.
When SHDN is high, the MAX608 enters shutdown
mode. In this mode, the internal biasing circuitry is
turned off (including the reference), and V OUT falls to
a diode drop below V IN (due to the DC path from the
input to the output). In shutdown mode, the supply
current drops to less than 5µA. SHDN is a TTL/CMOS
logic-level input. Connect SHDN to GND for normal
operation.
Practical inductor values range from 10µH to 300µH.
22µH is a good choice for most applications. In applications with large input/output differentials, the IC’s output-current capability will be much less when the inductance value is too low, because the IC will always operate
in discontinuous mode. If the inductor value is too low, the
8
Determining the Inductor (L)
_______________________________________________________________________________________
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
3.5
1.5
RSENSE = 25mΩ
RSENSE = 35mΩ
1.0
RSENSE = 50mΩ
0.5
RSENSE = 100mΩ
MAXIMUM OUTPUT CURRENT (A)
MAXIMUM OUTPUT CURRENT (A)
VOUT = 3.3V
L = 22µH
MAX608
2.0
VOUT = 5V
L = 22µH
3.0
RSENSE = 20mΩ
2.5
RSENSE = 25mΩ
2.0
RSENSE = 35mΩ
1.5
1.0
RSENSE = 50mΩ
0.5
RSENSE = 100mΩ
0
0
2.0
2.5
3.0
INPUT VOLTAGE (V)
2
3.5
Figure 4a. Maximum Output Current vs. Input Voltage
(VOUT = 3.3V)
3.5
VOUT = 12V
L = 22µH
3.0
RSENSE = 20mΩ
RSENSE = 25mΩ
2.5
RSENSE = 35mΩ
2.0
1.5
1.0
RSENSE = 50mΩ
0.5
5
Figure 4b. Maximum Output Current vs. Input Voltage
(VOUT = 5V)
MAXIMUM OUTPUT CURRENT (A)
MAXIMUM OUTPUT CURRENT (A)
3.5
3
4
INPUT VOLTAGE (V)
VOUT = 15V
L = 22µH
3.0
RSENSE = 20mΩ
RSENSE = 25mΩ
2.5
RSENSE = 35mΩ
2.0
1.5
1.0
RSENSE = 50mΩ
0.5
RSENSE = 100mΩ
0
2
4
6
8
INPUT VOLTAGE (V)
10
RSENSE = 100mΩ
0
12
2
4
6
8
10
12
INPUT VOLTAGE (V)
14
16
Figure 4c. Maximum Output Current vs. Input Voltage
(VOUT = 12V)
Figure 4d. Maximum Output Current vs. Input Voltage
(VOUT = 15V)
current will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time
for the switch (tON(min)) is approximately 2µs; select an
inductor that allows the current to ramp up to ILIM.
Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for
use with high switching frequencies. Make sure the
inductor’s saturation current rating (the current at which
the core begins to saturate and the inductance starts to
fall) exceeds the peak current rating set by R SENSE.
However, it is generally acceptable to bias the inductor
into saturation by approximately 20% (the point where
the inductance is 20% below the nominal value). For
highest efficiency, use a coil with low DC resistance,
preferably under 20mΩ. To minimize radiated noise,
use a toroid, a pot core, or a shielded coil.
Table 1 lists inductor suppliers and specific recommended inductors.
The standard operating circuits use a 22µH inductor.
If a different inductance value is desired, select L such
that:
VIN(max) x 2µs
L ≥ —————----—-ILIM
Larger inductance values tend to increase the start-up
time slightly, while smaller inductance values allow the
coil current to ramp up to higher levels before the
switch turns off, increasing the ripple at light loads.
_______________________________________________________________________________________
9
MAX608
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
Power Transistor Selection
Use an N-channel MOSFET power transistor with the
MAX608.
Use logic-level or low-threshold N-FETs to ensure the
external N-channel MOSFET (N-FET) is turned on completely and that start-up occurs. N-FETs provide the
highest efficiency because they do not draw any DC
gate-drive current.
When selecting an N-FET, some important parameters
to consider are the total gate charge (Qg), on-resistance (rDS(ON)), reverse transfer capacitance (CRSS),
maximum drain to source voltage (VDS max), maximum
gate to source voltage (VGS max), and minimum threshold voltage (VTH min).
Qg takes into account all capacitances associated with
charging the gate. Use the typical Qg value for best
results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be
50nC or less. With larger numbers, the EXT pins may
not be able to adequately drive the gate. The EXT
rise/fall time varies with different capacitive loads as
shown in the Typical Operating Characteristics.
The two most significant losses contributing to the
N-FET’s power dissipation are I2R losses and switching
losses. Select a transistor with low r DS(ON) and low
CRSS to minimize these losses.
Determine the maximum required gate-drive current
from the Qg specification in the N-FET data sheet.
Select an N-FET with a BVDSS > VOUT, BVGSS > VOUT,
and a minimum VTH of 0.5V below the minimum input
voltage.
When using a power supply that decays with time
(such as a battery), the N-FET transistor will operate in
its linear region when the voltage at EXT approaches
the threshold voltage of the FET, dissipating excessive
power. Prolonged operation in this mode may damage
the FET. To avoid this condition, make sure VEXT is
above the VTH of the FET, or use a voltage detector
(such as the MAX8211) to put the IC in shutdown mode
once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input
voltages can also cause this condition.
The MAX608’s maximum allowed switching frequency
during normal operation is 300kHz. However, at startup, the maximum frequency can be 500kHz, so the
maximum current required to charge the N-FET’s gate
is f(max) x Qg(typ). Use the typical Qg number from the
transistor data sheet. For example, the MMFT3055EL
has a Qg(typ) of 7nC (at VGS = 5V), therefore the current required to charge the gate is:
10
IGATE (max) = (500kHz) (7nC) = 3.5mA.
Figure 2a’s application circuit uses a 4-pin MMFT3055EL
surface-mount N-FET that has 150mΩ on-resistance with
4.5V VGS, and a guaranteed VTH of less than 2V. Figure
2c’s application circuit uses an Si6426DQ logic-level NFET with a threshold voltage (VTH) of 1V.
Diode Selection
The MAX608’s high switching frequency demands a
high-speed rectifier. Schottky diodes such as the
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
peak current limit set by RSENSE, and that its breakdown voltage exceeds V OUT . For high-temperature
applications, Schottky diodes may be inadequate due
to their high leakage currents; high-speed silicon
diodes such as the MUR105 or EC11FS1 can be used
instead. At heavy loads and high temperatures, the
benefits of a Schottky diode’s low forward voltage may
outweigh the disadvantage of high leakage current.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The
product of the peak inductor current and the output filter
capacitor’s ESR determines the amplitude of the ripple
seen on the output voltage. Two OS-CON 100µF, 16V
output filter capacitors in parallel with 35mΩ of ESR each
typically provide 75mV ripple when stepping up from 2V
to 5V at 500mA (Figure 2a). Smaller-value and/or higherESR capacitors are acceptable for light loads or in applications that can tolerate higher output ripple.
Since the output filter capacitor’s ESR affects efficiency, use low-ESR capacitors for best performance. See
Table 1 for component selection.
Input Bypass Capacitors
The input bypass capacitor (C1) reduces peak currents
drawn from the voltage source and also reduces noise
caused by the switching action of the MAX608 at the
voltage source. The input voltage source impedance
determines the size of the capacitor required at the
OUT input. As with the output filter capacitor, a low-ESR
capacitor is recommended. For output currents up to
1A, 150µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable.
Bypass the IC with a 0.1µF ceramic capacitor (C2)
placed as close as possible to the OUT and GND pins.
Reference Capacitor
Bypass REF with a 0.1µF capacitor (C3). REF can
source up to 100µA of current for external loads.
______________________________________________________________________________________
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
Surface Mount
Through Hole
INDUCTORS
CAPACITORS
Sumida
CD54 series
CDR125 series
Coiltronics
CTX20 series
Coilcraft
DO3316 series
DO3340 series
Matsuo
267 series
Sprague
595D series
AVX
TPS series
Sanyo
OS-CON series
Sumida
RCH855 series
RCH110 series
Sanyo
OS-CON series
Nichicon
PL series
Feed-Forward Capacitor
When adjusting the output voltage, it may be necessary
to parallel a 47pF to 220pF capacitor across R2, as
shown in Figures 2 and 3. Choose the lowest capacitor
value that insures stability; high capacitance values
may degrade line regulation.
__________Applications Information
Starting Up Under Load
The Typical Operating Characteristics show the Start-Up
Voltage vs. Load Current graphs for 5V and 12V output
voltages. These graphs depend on the type of power
switch used. The MAX608 is not designed to start up
under full load with low input voltages.
Layout Considerations
Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is
essential. Protect sensitive analog grounds by using a
star ground configuration. Minimize ground noise by
connecting GND, the input bypass capacitor ground
lead, and the output filter capacitor ground lead to a
single point (star ground configuration). Also, minimize
lead lengths to reduce stray capacitance, trace resistance, and radiated noise. Place input bypass capacitor C2 as close as possible to OUT and GND.
If an external resistor divider is used (Figures 2 and
3), the trace from FB to the resistors must be
extremely short.
TRANSISTORS
Siliconix
Si9410DY
Si4410DY
Si6426DQ
Si6946DQ
Motorola
MTP3055EL
MTD20N03HDL
MMFT3055ELT1
DIODES
Central Semiconductor
CMPSH-3
CMPZ5240
Nihon
EC11 FS1 series (highspeed silicon)
Motorola
MBRS1100T3
MMBZ5240BL
Motorola
1N5817–1N5822
MUR105 (high-speed
silicon)
SUPPLIER
PHONE
FAX
AVX
USA: (803) 448-9411
(803) 448-1943
Central
Semiconductor
USA: (516) 435-1110
(516) 435-1824
Coilcraft
USA: (708) 639-6400
(708) 639-1469
Coiltronics
USA: (407) 241-7876
(407) 241-9339
Matsuo
USA: (714) 969-2491
Japan: 81-6-337-6450
(714) 960-6492
81-6-337-6456
Motorola
USA: (800) 521-6274
(602) 952-4190
Nichicon
USA: (708) 843-7500
(708) 843-2798
Nihon
USA: (805) 867-2555
(805) 867-2556
Sanyo
USA: (619) 661-6835
Japan: 81-7-2070-1005
(619) 661-1055
81-7-2070-1174
Siliconix
USA: (800) 554-5565
(408) 970-3950
Sprague
USA: (603) 224-1961
(603) 224-1430
Sumida
USA: (708) 956-0666
Japan: 81-3-3607-5111
(708) 956-0702
81-3-3607-5144
______________________________________________________________________________________
11
MAX608
PRODUCTION
MAX608
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
___________________Chip Topography
EXT
OUT
CS
0.126"
(3.200mm)
GND
AGND
FB
SHDN
REF
0.080"
(2.032mm)
TRANSISTOR COUNT: 501
SUBSTRATE CONNECTED TO OUT
________________________________________________________Package Information
DIM
D
0°-8°
A
0.101mm
0.004in.
e
B
A1
E
12
C
H
L
Narrow SO
SMALL-OUTLINE
PACKAGE
(0.150 in.)
A
A1
B
C
E
e
H
L
INCHES
MAX
MIN
0.069
0.053
0.010
0.004
0.019
0.014
0.010
0.007
0.157
0.150
0.050
0.244
0.228
0.050
0.016
DIM PINS
D
D
D
8
14
16
MILLIMETERS
MIN
MAX
1.35
1.75
0.10
0.25
0.35
0.49
0.19
0.25
3.80
4.00
1.27
5.80
6.20
0.40
1.27
INCHES
MILLIMETERS
MIN MAX
MIN
MAX
0.189 0.197 4.80
5.00
0.337 0.344 8.55
8.75
0.386 0.394 9.80 10.00
______________________________________________________________________________________
21-0041A