EVALUATION KIT AVAILABLE
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
General Description
Features
The MAX8627 step-up converter is a high-efficiency,
low-quiescent current, synchronous boost converter
with True Shutdown™ and inrush current limiting. The
MAX8627 generates any boosted output voltage from
3V to 5V from either a 2-cell NiMH/NiCd or a single-cell
Li+/Li polymer battery.
Quiescent current is only 20µA (typ), and at light loads
the converter pulses only as needed for best efficiency.
At higher loads, PWM mode maintains fixed 1MHz
operation for lowest noise and ripple.
o 1MHz PWM Switching Frequency
The MAX8627 includes an internal soft-start to limit
inrush current to a maximum of 500mA. Additional features include True Shutdown, internal compensation,
and adjustable current limit. The MAX8627 is available
in a tiny 3mm x 3mm TDFN package and is ideal for
use in handheld devices such as DSCs, PDAs, and
smartphones.
o Internal Synchronous Rectifier
o True Shutdown Output
o Up to 95% Efficiency
o 1.0A Guaranteed Output Current
o Soft-Start Eliminates Inrush Current
o 20µA (typ) Quiescent Current
o 0.1µA Logic-Controlled Shutdown
o Internal Compensation
o Adjustable Current Limit
o Low-Noise Antiringing Feature
o Tiny 14-Pin, 3mm x 3mm, TDFN Package
Applications
DSC Motors and Backup Power
Ordering Information
Microprocessor/DSP Core Power
Cellphones, PDAs, MP3 Players
PART
Portable Handheld Devices
MAX8627ETD+
Visit www.maximintegrated.com/products/patents for
product patent marking information.
True Shutdown is a trademark of Maxim Integrated Products, Inc.
PINPACKAGE
14 TDFN-EP*
3mm x 3mm
LX
BATT
ON
3
POUT
ON
OFF
8, 9
6,7
C3
OUTS
ILIM
AGND
PG
PG
LX
LX
Pin Configuration
14
13
12
11
10
9
8
OUTPUT
3V TO 5V
UP TO 1A
C4
MAX8627
2
R2
1
GND
PGND
10, 11
1
2
3
4
5
6
POUT
R3
+
BATT
FB
MAX8627ETD+
R1
BATT
ILIM
14
FB
13
OUTS
ON
R4
AGND
GND
12
AAQ
7
POUT
4, 5
T1433-2
*EP = Exposed pad.
+Denotes lead-free package.
TOP VIEW
L1
C2
TOP
MARK
Note: The device operates in the -40°C to +85°C extended operating temperature range.
Typical Operating Circuit
INPUT BATTERY
2.5V TO 4.2V
C1
PKG
CODE
TDFN 3mm x 3mm
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
19-3995; Rev 0; 3/06
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
ABSOLUTE MAXIMUM RATINGS
Continuous Power Dissipation (TA = +70°C)
14-Pin TDFN 3mm x 3mm
(derate 18.2mW/°C above +70°C) .............................1454mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Note 1: LX has internal clamp diodes to the IC internal power node VPWR (where VPWR is the higher of BATT or POUT) and PG.
Applications that forward bias these diodes should take care not to exceed the device’s power-dissipation limits.
OUTS, BATT to GND ................................................-0.3V to +6V
LX Current (Note 1) ...............................................................3.5A
AGND, PG to GND ................................................-0.3V to +0.3V
POUT to OUTS ......................................................-0.3V to +0.3V
FB, ILIM, ON to
GND.....0.3V to the higher of (VOUTS + 0.3V) and (VBATT + 0.3V)
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
(VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, VILIM = GND, TA = -40°C to +85°C, typical values are at TA = +25°C, unless otherwise
noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
5.5
V
GENERAL
Operating Input Voltage Range
(Note 1)
Minimum Startup Voltage
No load (Note 1)
0.9
1.2
Maximum Startup Current Limit
Shutdown, ON = GND
Supply Current
1.5
0.5
No load, no switching
TA = +25°C
0.1
1
TA = +85°C
0.2
TA = 0°C to +85°C
20
30
TA = -40°C (Note 2)
20
35
No load, switching
V
A
µA
20
OSCILLATOR
Switching Frequency
0.95
1.0
Maximum Duty Cycle
82.5
87.0
Output Voltage Adjust Range
3.0
Startup Switching Frequency
1.05
2.0
MHz
%
5.2
No load
FB Load Regulation
0A to 1A output current load step
-30
mV/A
FB Line Regulation
VBATT = 2.7V to 3V, output current = 0.5A
+20
mV
FB Input Leakage Current
TA = +25°C
VFB = 1.2V,
VOUTS = VPOUT = VBATT = 5.5V TA = +85°C
Idle Mode Trip Level
-50
1.015
-10
+50
-10
Low level
High level
1.025
V
FB Regulation Voltage
ILIM Dual Mode™ Threshold
1.005
MHz
0.25
0.45
(Note 3)
50
V
nA
V
mA
DC-DC SWITCHES
n-Channel On-Resistance
0.15
0.25
Ω
p-Channel On-Resistance
0.15
0.25
Ω
17
30
Ω
3.5
3.7
Damping Switch On-Resistance
n-Channel Current limit
VILIM = 0V
VILIM = 0.6V
3.2
1.0
A
Dual Mode is a trademark of Maxim Integrated Products, Inc.
2
Maxim Integrated
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
ELECTRICAL CHARACTERISTICS (continued)
(VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, VILIM = GND, TA = -40°C to +85°C, typical values are at TA = +25°C, unless otherwise
noted.)
PARAMETER
CONDITIONS
MIN
p-Channel Turn-Off Current
TYP
MAX
10
POUT Leakage Current
VLX = 0V, VPOUT = VBATT = 5.5V
LX Leakage Current
VLX = 0V and VPOUT = 5.5V or VLX =
5.5V and VOUTS = VPOUT = 0V
Soft-Start Interval
Output current = 0.5A
TA = +25°C
0.1
TA = +85°C
0.2
TA = +25°C
0.1
TA = +85°C
0.2
Overload Protection Fault Delay
UNITS
mA
1
1
µA
µA
5.25
ms
65
ms
LOGIC INPUTS
ON Input Low Level
ON Input High Level
ON, Input Leakage Current
Thermal Shutdown
1.5V < VPOUT = VOUTS = VBATT ≤ 1.8V
0.2
1.8V < VPOUT = VOUTS = VBATT ≤ 5.5V
0.5
1.5V < VPOUT = VOUTS = VBATT ≤ 1.8V
VPOUT 0.2
1.8V < VPOUT = VOUTS + VBATT ≤ 5.5V
1.6
VOUTS = VPOUT = VBATT = 5.5V,
ON = 0V or ON = 5.5V
V
V
TA = +25°C
0.01
TA = +85°C
0.02
+160
1
µA
°C
Note 1: The MAX8627 is powered from OUTS. Once started, the IC operates down to 0.9V.
Note 2: Specifications to -40°C are guaranteed by design and not production tested.
Note 3: The idle-mode current threshold is the transition point between fixed-frequency PWM operation and idle-mode operation.
The specification is given in terms of output load current for an inductor value of 1µH. For a step-up converter, the idlemode transition varies with the input-to-output voltage ratio.
Maxim Integrated
3
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Typical Operating Characteristics
(Circuit of Figure 1, VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, TA = +25°C, unless otherwise noted.)
40
VBATT = 2.4V
40
30
20
10
10
VBATT = 1.8V
1
10
100
MAX8627 toc03
VPOUT = 3.3V
2.0
1.5
1.0
VPOUT = 5V
0
0.01
1000
2.5
0.5
0
0.1
0.1
1
10
100
1.0
1000
2.0
3.0
4.0
5.0
LOAD CURRENT (mA)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
OUTPUT VOLTAGE vs. LOAD CURRENT
NO-LOAD INPUT CURRENT
vs. INPUT VOLTAGE WITH 3.3V OUTPUT
NO-LOAD INPUT CURRENT
vs. INPUT VOLTAGE WITH 5V OUTPUT
VBATT = 3.0V
5.02
VBATT = 2.4V
5.00
VBATT = 1.8V
4.98
TA = +85°C
60
50
40
30
TA = +25°C
20
4.96
4.94
10
4.92
0
0.01
0.1
1
10
100
1.5
2.0
2.5
3.0
200
2.5
STARTUP VOLTAGE (V)
300
TA = +25°C
2.0
1.5
1.0
TA = 40°C
0.5
TA = 40°C
TA = +85°C
1.5
2.5
3.5
INPUT VOLTAGE (V)
4.5
5.5
2
1
3
4
5
6
5
4
3
2
1
0
0
TA = 40°C
SOFT-START TIME vs. LOAD CURRENT
TA = +25°C
100
TA = +25°C
INPUT VOLTAGE (V)
3.0
MAX8627 toc07
TA = +85°C
60
3.5
STARTUP VOLTAGE vs. LOAD CURRENT
WITH 5V OUTPUT
400
80
0
SHUTDOWN CURRENT
vs. INPUT VOLTAGE
500
TA = +85°C
100
20
INPUT VOLTAGE (V)
600
120
40
LOAD CURRENT (mA)
700
140
TA = 40°C
1.0
1000
R1 = 2MΩ, R2 = 499kΩ
OUTPUT ONLY LOADED WITH THE FB
RESISTOR-DIVIDER NETWORK.
160
MAX8627 toc09
VBATT = 3.6V
5.04
70
INPUT CURRENT (μA)
VBATT = 4.2V
5.06
180
SOFT-START TIME (ms)
5.08
R1 = 1.15MΩ, R2 = 499kΩ
OUTPUT ONLY LOADED WITH THE FB
RESISTOR-DIVIDER NETWORK.
80
INPUT CURRENT (μA)
5.10
MAX8627 toc05
90
MAX8627 toc04
5.12
OUTPUT VOLTAGE (V)
VBATT = 3.0V
50
20
0.01
SHUTDOWN CURRENT (nA)
VBATT = 3.6V
60
30
0
4
70
3.0
MAX8627 toc06
VBATT = 1.8V
VBATT = 1.5V
50
80
3.5
MAXIMUM LOAD CURRENT (A)
VBATT = 2.4V
70
VBATT = 4.2V
90
EFFICIENCY (%)
EFFICIENCY (%)
80
60
100
MAX8627 toc02
VBATT = 3.0V
MAX8627 toc08
90
MAX8627 toc01
100
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
EFFICIENCY vs. LOAD CURRENT
WITH 5V OUTPUT
EFFICIENCY vs. LOAD CURRENT
WITH 3.3V OUTPUT
0
0
200
400
600
LOAD CURRENT (mA)
800
1000
0
0.1
0.2
0.3
0.4
0.5
LOAD CURRENT (A)
Maxim Integrated
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, TA = +25°C, unless otherwise noted.)
SOFT-START TIME vs. INPUT VOLTAGE
3.5
3.0
2.5
2.0
1.5
1.0
MAX8627 toc11
4.0
4.0
3.5
PEAK INDUCTOR CURRENT (A)
4.5
SOFT-START TIME (ms)
PEAK INDUCTOR CURRENT vs. VILIM
MAX8627 toc10
5.0
3.0
2.5
2.0
1.5
1.0
0.5
0.5
0
0
0.5
1.5
2.5
3.5
4.5
5.5
0.45
INPUT VOLTAGE (V)
0.65
0.85
1.05
1.25
VILIM (V)
HEAVY LOAD SWITCHING WAVEFORMS
LIGHT-LOAD SWITCHING WAVEFORMS
MAX8627 toc12
MAX8627 toc13
100mV/div
(AC-COUPLED)
VPOUT
100mV/div
(AC-COUPLED)
VPOUT
2V/div
2V/div
VLX
VLX
0
0
1A/div
ILI
1A/div
ILI
0
0
1µs/div
40µs/div
LOAD TRANSIENT RESPONSE
LINE TRANSIENT RESPONSE
MAX8627 toc14
MAX8627 toc15
2V/div
VPOUT
100mV/div
(AC-COUPLED)
VBATT
0
VPOUT
ILOAD
100mV/div
(AC-COUPLED)
500mA/div
0
20µs/div
Maxim Integrated
100µs/div
5
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, TA = +25°C, unless otherwise noted.)
STARTUP WAVEFORMS WITH 100mA LOAD
STARTUP WAVEFORMS WITH NO LOAD
MAX8627 toc17
MAX8627 toc16
VON
VON
2V/div
0
2V/div
0
2V/div
2V/div
VPOUT
VPOUT
0
ILX
ILX
500mV/div
500mV/div
400µs/div
400µs/div
SWITCHING FREQUENCY vs. TEMPERATURE
BODE PLOT WITH 2 x 22µF CERAMIC
OUTPUT CAPACITORS, 500mA LOAD
MAX8627 toc19
MAX8627 toc18
1006
1004
40
1002
GAIN (dB)
1000
998
996
GAIN
30
180
20
150
10 PHASE
-10
48-DEG PHASE MARGIN
94kHz
-30
992
120
90
60
-20
994
7dB GAIN
MARGIN
0
PHASE (DEG)
SWITCHING FREQUENCY (kHz)
0
30
0
-40
990
-40
-15
10
35
1k
85
60
10k
100k
1M
FREQUENCY (Hz)
TEMPERATURE (°C)
BODE PLOT WITH 2 x 47µF TANTALUM OUTPUT
CAPACITORS (130mΩ ESR), 500mA LOAD
MAX8627 toc20
40
30
10 PHASE
125 - DEG PHASE MARGIN
150
27kHz
120
13dB GAIN MARGIN
90
-10
60
-20
30
-30
0
PHASE (DEG)
GAIN (dB)
180
GAIN
20
-40
1k
10k
100k
1M
FREQUENCY (Hz)
6
Maxim Integrated
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Pin Description
PIN
NAME
1
GND
2
FB
Voltage Feedback Input. Connect FB to the center of an external feedback network between OUTS
and GND (see the Setting the Output Voltage section). FB regulates to 1.015V (typ).
3
ON
Active-High Enable Input. Connect ON to BATT or logic high for normal operation. Connect ON to
GND or logic low for True Shutdown mode.
4, 5
BATT
Supply Voltage Input. Connect to the battery or a supply from 1.5V to 5.5V. Connect two 22µF
ceramic capacitors from BATT to PG.
6, 7
POUT
Power Output. Connect two 22µF ceramic capacitors from POUT to PG (see the Capacitor Selection
section).
8, 9
10, 11
LX
PG
12
AGND
13
ILIM
14
OUTS
—
EP
Maxim Integrated
FUNCTION
Analog Ground. Connect to PG and AGND.
Inductor Connection. LX is high impedance in shutdown.
Power Ground. Connect to GND and AGND.
Analog Ground. Connect to GND and PG.
n-Channel Current-Limit Control. For the maximum current limit of 3.5A, connect ILIM to GND. For
lower current-limit settings, connect ILIM to a resistor-divider from POUT to GND (see the Setting the
Current Limit section).
IC Power Input. Supplied from the output. Connect OUTS to POUT.
Exposed Pad. Connect EP to GND. This does not remove the requirement for a proper ground
connection to GND.
7
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
L1
1µH
INPUT: 1.5V TO 5.5V
C1
22µF
C2
22µF
LX
BATT
OUTPUT:
3V TO 5V, UP TO 1A
ON
OFF
POUT
ON
AGND
C3
22µF
MAX8627
R1
C4
22µF
OUTS
R3
FB
ILIM
GND
PG
R2
R4
Figure 1. Typical Applications Circuit with an Adjustable Output Voltage and Adjustable Current Limit
ON
2.7V
OUTS
+
ILIM
MAX8627
UVLO
POUT
ON
ON
STARTUP
OSCILLATOR
ON
BATT
CONTROL
ON
DAMPING
SWITCH
1MHz
OSCILLATOR
GND
LX
REFERENCE
PG
FB
Figure 2. Functional Diagram
8
Maxim Integrated
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Detailed Description
The MAX8627 is a current-mode step-up converter that
uses a fixed-frequency PWM architecture with True
Shutdown. Consuming only 20µA of quiescent current,
the MAX8627 is highly efficient, with an internal switch
and synchronous rectifier. Shutdown reduces the
quiescent current to less than 1µA. Low quiescent current and low noise make this device ideal for powering
portable equipment.
The MAX8627 step-up DC-to-DC switching converter
typically generates a 3V to 5V output voltage from a
1.5V to 4.2V battery input voltage. The IC operates in
bootstrapped mode with the output powering the IC
once the output voltage is equal to, or exceeds, 2.7V.
The default current limit is set at 3.5A to deliver 1A at
5V with an Li+ battery, or 500mA at 5V using a 2-cell
NiCd/NiMH battery. The current limit may be lowered
using an external resistor at ILIM to allow for smaller
components in lower power applications. Internal softstart limits the inrush current to less than 500mA under
no-load conditions during startup.
The MAX8627 switches at an internally set frequency of
1MHz allowing for tiny external components. Internal
compensation further reduces the external component
count in cost and space-sensitive applications. The
MAX8627 is optimized for use in DSC and other applications requiring low quiescent current for maximum
battery life. Figure 1 shows the typical applications
circuit. Figure 2 gives the functional diagram.
DC-DC Converter
The MAX8627 uses a current-mode PWM control
scheme. The voltage difference between FB and an
internal 1.01V reference generates an error signal that
programs the peak inductor current to regulate the
output voltage. The default peak inductor current limit is
typically 3.5A. Inductor current is sensed across the
internal switch and summed with a slope-compensation
signal. The PWM comparator compares this summed
signal to the error amplifier output. At the beginning of
each clock cycle, the n-channel switch turns on until
the PWM comparator trips. During this time, inductor
current ramps up, storing energy in its magnetic field.
When the n-channel switch turns off, the internal
synchronous p-channel rectifier turns on. The inductor
releases the stored energy as the current ramps down
and provides energy to the output.
Maxim Integrated
The device operates in PWM when driving medium to
heavy loads. As the load current decreases and crosses
the low-power idle mode threshold, the PWM comparator
and oscillator are disabled. In this low-power idle mode,
switching occurs only as needed to service the output.
This improves the efficiency for light loads and the IC
consumes only 20µA under no-load conditions. At light
loads, the output ripple has a frequency component that
varies with load current. The threshold for entering the
low-power mode is determined by sensing the voltage
drop across the internal switch and comparing it to an
internally generated reference level. This threshold is
approximately 50mA with a 3.6V input and 5V output.
When switching in low-power mode, the inductor
current terminates at zero for each switching cycle. When
operating in this manner, the inductor current is called
discontinuous. In older DC-DC converters, radiated noise
may be higher when inductor current is discontinuous,
because of ringing at the LX switch. The MAX8627 features an internal damping switch to minimize ringing at LX
when inductor current is discontinuous. The damping
switch places an impedance across the inductor and
supplies a path to dissipate the resonant energy in the
inductor and capacitor to damp the ringing at the LX.
The damping switch has little effect on output voltage
ripple but does reduce EMI.
At higher loads, the MAX8627 operates in PWM mode.
Regulation is achieved by modulating the MOSFET
switch pulse to control the amount of power transferred
per cycle. Switching harmonics generated by fixedfrequency operation are consistent and easily filtered.
This is important in noise-sensitive applications.
Load-Transient Response/Voltage
Positioning
The MAX8627 matches the load regulation to the voltage droop seen during load transients. This is sometimes called voltage positioning. Benefits include lower
peak-to-peak output-voltage deviation for a given load
step without requiring an increase in filter load capacitance. There is minimal voltage droop when transitioning
from a light load to full load and minimum overshoot
when going from full load to light load.
9
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
The term “positioning” refers to setting the output voltage to a level that is dependent on load current (see
Figure 3). At minimum load, the output voltage is set to
a slightly higher than nominal level. At full load, the output voltage is slightly lower than the nominal level. With
voltage positioning, the total voltage deviation during a
transient is significantly improved over traditional highgain control loops. Traditional high-gain loops use integrators that maximize gain at low frequencies to
provide tight DC-load regulation; however, due to the
capacitive element in the feedback loop, these highgain amplifiers typically take hundreds of microseconds to respond to a load step and return to steady
state. As a result, the voltage can droop by as much as
6% or more during the recovery time. In portable equipment where the output load can change frequently, and
the amount of output capacitance that can fit is limited,
this can result in a wide short-term output fluctuation
(see Figure 4).
Voltage positioning on the MAX8627 allows up to 3%
(typ) of load regulation and no further transient droop
(Figures 3 and 4). Thus, during load transients the voltage delivered remains within specification more effectively than other regulators that might have tighter DC
accuracy. In systems with high-speed CPUs, thousands of system clock cycles can occur during the time
it takes a traditional high-gain loop to respond to a load
step. Consequently, 3% load regulation with no transient droop is better suited to such systems than a
power supply that may spec 1% DC load regulation,
but then exhibits 6% or more of transient droop during
load steps (see the Load Transient Response in the
Typical Operating Characteristics section).
True Shutdown
Connecting ON to GND or logic low places the
MAX8627 in shutdown mode and reduces supply current to 0.1µA. In shutdown, the control circuitry, internal
switching MOSFET, and synchronous rectifier turn off
and LX becomes high impedance. Connect ON to
BATT or logic high for normal operation.
The MAX8627 has an internal synchronous rectifier,
which allows for conversion efficiencies as high as
95%. In conventional boost circuits, the body diode of
the synchronous rectifier is forward biased in shutdown
and allows current flow from the battery to the output. If
the load cannot be shut down, an external switch is
required to avoid depleting the battery during shutdown. A proprietary design in the MAX8627 allows the
synchronous rectifier to provide True Shutdown with no
additional components. This allows the output to fall to
GND in shutdown and removes any connection
between the input and output.
Soft-Start
The MAX8627 has internal soft-start circuitry that eliminates inrush current at startup, reducing transients on
the input source. Soft-start is particularly useful for
higher impedance input sources, such as Li+ and alkaline cells. The soft-start duration is proportional to the
size of the output capacitor and load resistance with a
typical time of 5.25ms. See the Typical Operating
Characteristics section for plots of Soft-Start Time vs.
Load Current and Soft-Start Time vs. Input Voltage.
Inrush current is controlled during startup and initially
set to 500mA. After 1000 clock cycles, if the output voltage is not within regulation, the startup current limit is
OUTPUT VOLTAGE
vs. LOAD CURRENT
5.02
5.00
9%
OUTPUT VOLTAGE (V)
4.98
4.96
VIN = 4V
4.94
(a) HIGH-GAIN DC LOAD REGULATION WITH POOR TRANSIENT RESPONSE
4.92
4.90
4.88
4.86
VIN = 2.8V
4.84
4.82
3%
VIN = 1.8V
4.80
0
500
1000
1500
2000
(b) VOLTAGE POSITIONING WITH DC LOAD REGULATION
LOAD CURRENT (mA)
Figure 3. Load-Regulation Specification
10
Figure 4. Transient-Response Comparison
Maxim Integrated
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
incremented by 230mA. If after 13 increments, the output is still not in regulation, the MAX8627 latches off,
assuming a short-circuit overload condition exists on
the output. To clear the latched condition, cycle ON.
which sets peak-to-peak inductor current at 1/2 the DC
inductor current:
L=
Fault Protection
2 x VBATT x D x (1− D)
IOUT(MAX) x fSW
The MAX8627 has a fault-overload protection. After
soft-start, the device is set to detect an out-of-regulation state that could be caused by an overload. If the
output remains faulted for 65ms, then the MAX8627
latches off. Fault-detection circuitry is disabled during
soft-start. If short on the output exists before the
MAX8627 is turned ON, the converter completes the
soft-start sequence and latches off. The converter can
be reinitialized from a fault latch-off state by toggling
the ON pin or by cycling the input power.
where fSW is the switching frequency (1MHz), and D is
the duty factor given by D = 1 - ( VBATT / VOUT ).
Using L from the equation above results in a peak-topeak inductor current ripple of 0.5 x IOUT / (1 - D), and
a peak inductor current of 1.25 x IOUT / (1 - D). Ensure
the peak (saturation) current rating of the inductor
meets or exceeds this requirement.
The recommended inductance range for the MAX8627 is
1µH to 4.7µH. See Table 1 for recommended inductors.
BATT/Damping Switch
Capacitor Selection
The MAX8627 features an internal damping switch to
minimize ringing at LX caused by the resonant circuit
formed by the inductor and output capacitor in discontinuous conduction mode. This occurs at light loads.
The damping switch connects across the inductor
when the inductor energy is depleted and supplies a
path to dissipate the resonant energy. Damping LX
ringing does not change the output ripple but reduces
EMI.
Applications Information
Setting the Output Voltage
To set the output voltage to between 3V and 5V, connect FB to the center of an external resistor voltagedivider between OUTS and GND, as shown in Figure 1.
Select the value of R2 less than 500kΩ, and then calculate the value for R1 as follows:
⎛V
⎞
R1 = R2 x ⎜ OUT − 1⎟
V
⎝ FB
⎠
where VFB is the FB regulation voltage, 1.015V (typ).
Inductor Selection
In most step-up converter designs, a reasonable inductor
value can be derived from the following equation,
Output Capacitor
Output capacitors C3 and C4 in Figure 1 are required to
keep the output voltage ripple small and to ensure regulation loop stability. The output capacitors must have low
impedance at the switching frequency. Ceramic capacitors are highly recommended due to their small size and
low ESR. Make sure the output capacitors maintain their
capacitance over DC bias and the desired operating
temperature range. Ceramic capacitors with X5R or X7R
temperature characteristics generally perform well. Two
22µF ceramic capacitors in parallel are recommended.
Alternatively, two 47µF tantalum capacitors with 70mΩ or
lower ESR may be used.
Input Capacitor
Input capacitors C1 and C2 reduce the current peaks
drawn from the battery or input power source and reduce
switching noise in the IC. The impedance of the input
capacitors at the switching frequency should be kept
very low. Ceramic capacitors are highly recommended
due to their small size and low ESR. Make sure the input
capacitors maintain their capacitance over DC bias and
the desired operating temperature range. Ceramic
capacitors with X5R or X7R temperature characteristics
generally perform well. Two 22µF ceramic capacitors are
recommended.
Table 1. Recommended Inductors
PART
INDUCTANCE (µH)
RATED CURRENT (mA)
SIZE:
L (mm, typ) x W (mm, typ) x H (mm, max)
TOKO A918CY
1.0
3500
6.3 x 6.2 x 2
TOKO A997AS
1.5
2150
3.8 x 3.8 x 1.8
Maxim Integrated
11
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Setting the Current Limit
PC Board Layout and Routing
ILIM sets the current limit when the output reaches
regulation. It is different from the startup current limit
used during soft-start to control inrush current. For the
maximum current limit of 3.5A, connect ILIM to GND.
To set the current limit (ILIM) lower than 3.5A, connect
ILIM to a resistor-divider from POUT to GND as shown
in Figure 1. Note, however, that the idle-mode threshold
does not change with voltage setting on ILIM.
Set R3 between 30kΩ and 300kΩ, then calculate the
value of R4 as follows:
Good PC board layout is important to achieve optimal
performance from the MAX8627. Poor design can
cause excessive conducted and/or radiated noise.
Conductors carrying discontinuous currents and any
high-current path should be made as short and wide as
possible. Keep the feedback network (R1 and R2) very
close to the IC, preferably within 0.2in of the FB and
GND pins. Nodes with high dV/dt (switching nodes)
should be kept as small as possible and routed away
from FB. Connect the input and output capacitors as
close as possible to the IC. Refer to the MAX8627
evaluation kit for a PC board layout example.
⎛
⎞
VPOUT
R4 = R3 x ⎜
−1⎟
⎝ (ILIM + 0.64A ) × 0.2865Ω ⎠
Chip Information
PROCESS: BiCMOS
12
Maxim Integrated
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Package Information
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the
package regardless of RoHS status.
Maxim Integrated
13
MAX8627
Low VBATT, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
Package Information (continued)
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the
package regardless of RoHS status.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent
licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and
max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
14 ________________________________Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
© 2006 Maxim Integrated Products, Inc.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
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