MAXIM MAX8625A

19-1006; Rev 0; 3/08
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
The MAX8625A PWM step-up/down regulator is intended to power digital logic, hard disk drives, motors, and
other loads in portable, battery-powered devices such
as PDAs, cell phones, digital still cameras (DSCs), and
MP3 players. The MAX8625A provides either a fixed
3.3V or adjustable output voltage (1.25V to 4V) at up to
0.8A from a 2.5V to 5.5V input. The MAX8625A utilizes
a 2A peak current limit.
Maxim’s proprietary H-bridge topology* provides a
seamless transition through all operating modes without
the glitches commonly seen with other devices. Four
internal MOSFETs (two switches and two synchronous
rectifiers) with internal compensation minimize external
components. A SKIP input selects a low-noise, fixedfrequency PWM mode, or a high-efficiency skip mode
where the converter automatically switches to PFM
mode under light loads for best light-load efficiency.
The internal oscillator operates at 1MHz to allow for a
small external inductor and capacitors.
The MAX8625A features current-limit circuitry that shuts
down the IC in the event of an output overload. In addition, soft-start circuitry reduces inrush current during
startup. The IC also features True ShutdownTM, which
disconnects the output from the input when the IC is
disabled. The MAX8625A is available in a 3mm x 3mm,
14-pin TDFN package.
Applications
PDAs and Smartphones
DSCs and Camcorders
MP3 Players and Cellular Phones
Features
♦
♦
♦
♦
Four Internal MOSFET True H-Bridge Buck/Boost
Glitch-Free, Buck-Boost Transitions
Minimal Output Ripple Variation on Transitions
Up to 92% Efficiency
♦ 37µA (typ) Quiescent Current in Skip Mode
♦ 2.5V to 5.5V Input Range
♦ Fixed 3.3V or Adjustable Output
♦ 1µA (max) Logic-Controlled Shutdown
♦
♦
♦
♦
♦
♦
♦
True Shutdown
Output Overload Protection
Internal Compensation
Internal Soft-Start
1MHz Switching Frequency
Thermal-Overload Protection
Small 3mm x 3mm, 14-Pin TDFN Package
Ordering Information
PINPACKAGE
PART
TOP MARK
PKG
CODE
ABQ
T1433-2
14 TDFN-EP**
(3mm x 3mm)
MAX8625AETD+
Note: The device is specified over the -40°C to +85°C extended
temperature range.
+Denotes a lead-free package.
**EP = Exposed pad.
Battery-Powered Hard Disk Drive (HDD)
Pin Configuration
Typical Operating Circuit
IN
IN
GND
GND
OUT
OUT
REF
TOP VIEW
14
13
12
11
10
9
8
INPUT
2.7V TO 5.5V
LX1
IN
LX2
OUTPUT
3.3V
OUT
GND
MAX8625A
PWM
EP
+
SKIP
2
3
4
5
6
7
LX1
LX1
LX2
LX2
ON
SKIP
FB
SKIP
1
TDFN-EP
EP = EXPOSED PAD.
ON
OFF
MAX8625A
FB
REF
ON
*US Patent #7,289,119.
True Shutdown is a trademark of Maxim Integrated Products, Inc.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
1
MAX8625A
General Description
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
IN, OUT, SKIP, ON to GND ......................................-0.3V to +6V
REF, FB, to GND...............................................-0.3V, (IN + 0.3V)
LX2, LX1 (Note 1).........................................................±1.5ARMS
Continuous Power Dissipation (TA = +70°C)
Single-Layer Board (derate 18.5mW/°C
above TA = +70°C) ...................................................1482mW
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: LX1 and LX2 have internal clamp diodes to IN, PGND and OUT, PGND, respectively. Applications that forward bias these
diodes should take care not to exceed the device's power-dissipation limits.
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
(VIN = 3.6V, ON = SKIP = IN, FB = GND, VOUT = 3.3V, LX_ unconnected, CREF = C5 = 0.1µF to GND, Figure 4. TA = -40°C to +85°C.
Typical values are at TA = +25°C, unless otherwise noted.) (Note 2)
PARAMETER
Supply Range
UVLO Threshold
SYMBOL
CONDITIONS
VIN
UVLO
VIN rising, 60mV hysteresis
MIN
MAX
UNITS
2.5
TYP
5.5
V
2.20
2.49
V
22
mA
Quiescent Supply Current, FPWM
Mode, Switching
IIN
No load, VOUT = 3.2V
15
Quiescent Supply Current, Skip
Mode, Switching
IIN
SKIP = GND, no load
37
Quiescent Supply Current, No
Switching, Skip Mode
IIN
SKIP = GND, FB = 1.3V
35
45
Shutdown Supply Current
IIN
ON = GND, TA = +25°C
0.1
1
TA = +85°C
0.2
PWM mode, VIN = 2.5V to 5.5V
3.30
IOUT = 0 to 0.5A, VIN = 2.5V to 5.5V,
TA = -40°C to +85°C (Note 3)
Output Voltage Accuracy
(Fixed Output)
-1
3.28
Average skip voltage
3.285
Output Voltage Range
(Adjustable Output)
µA
%
V
-3
1.25
µA
V
+1
SKIP mode, valley regulation value
Load step +0.5A
µA
%
4.00
V
Maximum Output Current
VIN = 3.6V
0.80
A
Soft-Start
L = 3.3µH; COUT = C3 + C4 = 44µF
250
mA/ms
Load Regulation
IOUT = 0 to 500mA
0.1
%/mA
VIN = 2.5V to 5.5V
0.03
%/V
Line Regulation
OUT Bias Current
IOUT
VOUT = 3.3V
REF Output Voltage
VREF
VIN = 2.5V to 5.5V
REF Load Regulation
IREF = 10µA
FB Feedback Threshold
IOUT = 0 to full load, PWM mode; VIN = 2.5V
to 5.5V
2
VFB
3
1.244
1.25
µA
1.256
1
1.244
1.25
_______________________________________________________________________________________
V
mV
1.258
V
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
(VIN = 3.6V, ON = SKIP = IN, FB = GND, VOUT = 3.3V, LX_ unconnected, CREF = C5 = 0.1µF to GND, Figure 4. TA = -40°C to +85°C.
Typical values are at TA = +25°C, unless otherwise noted.) (Note 2)
PARAMETER
FB Dual-Mode Threshold
SYMBOL
CONDITIONS
VFBDM
75
0.1
VFB = 1.3V, TA = +85°C
0.01
VIH
2.5V < VIN < 5.5V
VIL
2.5V < VIN < 5.5V
ON Input Leakage Current
IIHL
1.6
2.5V < VIN < 5.5V, TA = +25°C
0.001
TA = +85°C
0.01
ISKIPH
VSKIP = 3.6V
ISKIPL
VSKIP = 0V
-2
-0.2
3
ILIMP
LX1 PMOS
1700
2000
LX1, LX2 Leakage Current
Out Reverse Current
Minimum TON
OSC Frequency
Thermal Shutdown
1
12
2300
100
Each MOSFET, TA = +25°C
RON
ILX1OFF
ISKIP
ILXLKG
ILXLKGR
0.05
Each MOSFET, VIN = 2.5V to 5.5V,
TA = -40°C to +85°C
100
Load increasing
300
VIN = VOUT = 5.5V, VLX1 = 0V to VIN,
VLX2 = 0V to VOUT, TA = +25°C
0.01
TA = +85°C
0.2
VIN = VLX1 = VLX2 = 0V, VOUT = 5.5V,
measure I (LX2), TA = +25°C
0.01
TA = +85°C
0.5
TONMIN
850
15°C hysteresis
1000
+165
µA
mA
Ω
mA
mA
1
1
25
FOSCPWM
µA
ms
125
SKIP = GND, load decreasing
V
0.1
0.2
SKIP = GND
µA
V
0.45
Fault Latch-Off Delay
Idle-Mode Current Threshold
(Note 4)
mV
125
ON, SKIP Input Low Voltage
Rectifier-Off Current Threshold
UNITS
100
ON, SKIP Input High Voltage
MOSFET On-Resistance
MAX
0.001
IFB
Peak Current Limit
TYP
VFB = 1.3V, TA = +25°C
FB Leakage Current
SKIP Input Leakage Current
MIN
µA
µA
%
1150
kHz
°C
Note 2: Devices are production tested at TA = +25°C. Specifications over the operating temperature range are guaranteed by
design and characterization.
Note 3: Limits are guaranteed by design and not production tested.
Note 4: 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 3.3µH. For the step-up mode, the idle-mode
transition varies with input to the output-voltage ratios.
_______________________________________________________________________________________
3
MAX8625A
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VIN = 3.6V, SKIP = GND, TA = +25°C, Figure 4, unless otherwise noted.)
SKIP-MODE EFFICIENCY
vs. INPUT VOLTAGE
60
50
VOUT = 3.3V
VIN = 2.7V
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
40
30
20
10
90
EFFICIENCY (%)
70
95
85
100mA
80
500mA
75
1
10
60
50
VOUT = 2.8V
VIN = 2.7V
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
40
20
VOUT = 3.3V
LOAD CURRENT = 100mA,
300mA, 500mA
65
10
0
2.0
1000
100
70
30
60
0.1
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
0.1
1
10
1000
100
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT
FPWM MODE (FIGURE 3)
OUTPUT VOLTAGE (3.3V INTERNAL FB)
vs. LOAD CURRENT
OUTPUT VOLTAGE (2.8V EXTERNAL FB)
vs. LOAD CURRENT (FIGURE 3)
80
1.5
2.0
1.5
DEVIATION (%)
60
50
VOUT = 3.45V
VIN = 2.7V
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
40
30
20
10
0
1.0
DEVIATION (%)
1.0
70
0.5
0
-0.5
1
10
0
-0.5
-1.0
-1.5
-1.5
1000
100
0.5
-1.0
VOUT = 3.3V
TA = +25°C, TA = -40°C, TA = +85°C,
-2.0
0.1
MAX8625A toc06
90
MAX8625A toc05
2.0
MAX8625A toc04
100
0.1
1
10
VOUT = 2.8V
TA = +25°C, TA = -40°C, TA = +85°C
-2.0
1000
100
0.1
1
10
1000
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH INTERNAL FB RESISTORS
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH EXTERNAL FB RESISTORS
SUPPLY CURRENT vs. INPUT VOLTAGE
WITH NO LOAD
3.31
3.30
3.29
3.28
LOAD: 500mA, VOUT = 3.3V
TA = +25°C, TA = -40°C, TA = +85°C
2.80
2.79
2.78
2.77
2.76
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE (V)
5.5
6.0
10
1
0.1
NO LOAD VOUT = 3.3V
LOAD: 500mA, VOUT = 2.8V
TA = +25°C, TA = -40°C, TA = +85°C (FIGURE 3)
0.01
2.75
3.27
FPWM MODE
SUPPLY CURRENT (mA)
2.81
OUTPUT VOLTAGE (V)
3.32
100
MAX8625A toc09
2.82
MAX8625A toc07
3.33
MAX8625A toc08
EFFICIENCY (%)
80
300mA
70
0
4
90
EFFICIENCY (%)
80
100
MAX8625A toc02
90
EFFICIENCY (%)
100
MAX8625A toc01
100
EFFICIENCY vs. LOAD CURRENT
FPWM MODE (FIGURE 3)
MAX8625A toc03
EFFICIENCY vs. LOAD CURRENT
SKIP AND FPWM MODES
OUTPUT VOLTAGE (V)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE (V)
5.5
6.0
2.0
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5.0
5.5
6.0
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
(VIN = 3.6V, SKIP = GND, TA = +25°C, Figure 4, unless otherwise noted.)
SWITCHING WAVEFORMS
VIN = 3V, LOAD = 500mA, VOUT = 3.3V
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
MAX8625A toc11
MAX8625A toc10
1000
MAXIMUM LOAD CURRENT (mA)
900
800
VOUT = 3.3V
700
VOUT
50mV/div
(AC-COUPLED)
VLX1
2V/div
600
500
VLX2
2V/div
400
300
ILX
500mA/div
200
100
0
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
1μs/div
6.0
INPUT VOLTAGE (V)
SWITCHING WAVEFORMS
VIN = 3.6V, LOAD = 500mA, VOUT = 3.3V
SWITCHING WAVEFORMS
VIN = 3.3V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc13
MAX8625A toc12
VOUT
50mV/div
(AC-COUPLED)
VOUT
50mV/div
(AC-COUPLED)
VLX1
2V/div
VLX1
2V/div
VLX2
2V/div
VLX2
2V/div
ILX
500mA/div
ILX
500mA/div
1μs/div
1μs/div
SKIP MODE
VIN = 3V, LOAD = 20mA,
VOUT = 3.288V
FPWM MODE
VIN = 3V, LOAD = 20mA,
VOUT = 3.308V
MAX8625A toc15
MAX8625A toc14
VOUT
20mV/div
(AC-COUPLED)
OUT
20mV/div
(AC-COUPLED)
CH1 = VLX1
2V/div
VLX1
2V/div
CH2 = VLX2
2V/div
VLX2
2V/div
ILX
500mA/div
10μs/div
ILX
500mA/div
1μs/div
_______________________________________________________________________________________
5
MAX8625A
Typical Operating Characteristics (continued)
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA = +25°C, Figure 4, unless otherwise noted.)
STARTUP WAVEFORMS
VIN = 3.6V, LOAD = 5Ω, VOUT = 3.288V
STARTUP WAVEFORMS (FIGURE 3)
VIN = 3.6V, LOAD = 30Ω, VOUT = 1.5V
MAX8625A toc16
MAX8625A toc17
SHDN
2V/div
SHDN
2V/div
VOUT
20mV/div
VOUT
500mA/div
ILX
500mA/div
ILX
500mA/div
IBATT
500mA/div
IBATT
100mA/div
2ms/div
2ms/div
LOAD TRANSIENT
VOUT = 3.3V
LINE TRANSIENT
VOUT = 3.3V, LOAD = 5.5Ω,
VIN RAMP 3V TO 4V
MAX8625A toc19
MAX8625A toc18
VOUT
100mV/div
(DC OFFSET = 3.3V)
CH2 = VOUT
50mV/div
(AC-COUPLED)
IBATT
250mA/div
CH1 = VIN
500mV/div
3V OFFSET
ILX
500mA/div
1ms/div
BODE PLOT
GAIN AND PHASE vs. FREQUENCY
OSCILLATOR FREQUENCY
vs. TEMPERATURE
GAIN
144
1.04
20
108
10
72
0
36
PHASE
-10
0
-20
-36
-30
-72
-108
VIN = 3.6
VOUT = 3.3V
LOAD = 200mA
-40
-50
-144
-180
-60
1
10
100
FREQUENCY (kHz)
6
1.06
1000
OSCILLATOR FREQUENCY (MHz)
30
180
PHASE (DEG)
MAX8625A toc20
MAX8625A toc21
400μs/div
40
GAIN (dB)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
1.02
1.00
0.98
0.96
0.94
0.92
0.90
-40
-20
0
20
40
60
80
100
TEMPERATURE (°C)
_______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
(VIN = 3.6V, SKIP = GND, TA = +25°C, Figure 4, unless otherwise noted.)
MINIMUM STARTUP VOLTAGE
vs. TEMPERATURE
REFERENCE vs. TEMPERATURE
NO LOAD
1.27
2.42
REFERENCE (V)
MINIMUM STARTUP VOLTAGE (V)
2.44
MAX8625A toc23
VOUT = 3.3V, NO LOAD
2.46
1.28
MAX8625A toc22
2.48
2.40
2.38
2.36
1.26
1.25
1.24
2.34
2.32
VOUT = 3.3V
VIN = 3.0V,
3.6V,
4.2V,
5.0V
1.23
2.30
2.28
1.22
-50
-25
0
25
50
75
100
-20
0
20
40
60
80
TEMPERATURE (°C)
REFERENCE vs. TEMPERATURE
WITH 300mA LOAD
SHUTDOWN DUE TO OVERLOAD
VIN = 3.6V, VOUT = 3.288V
100
MAX8625A toc25
MAX8625A toc24
1.28
1.27
REFERENCE (V)
-40
TEMPERATURE (°C)
VLX2
2V/div
1.26
VLX2
2V/div
1.25
1.24
VOUT
500mV/div
VOUT = 3.3V
VIN = 3.0V,
3.6V,
4.2V,
5.0V
1.23
ILX
500mA/div
1.22
-40
-20
0
20
40
60
80
100μs/div
100
TEMPERATURE (°C)
BOOST-TO-BUCK TRANSITION
FPWM MODE VIN = 3.6V, VOUT = 3.288V
MAX8625A toc26
VOUT
100mV/div
AC-COUPLED
VIN
1V/div
DC OFFSET = 3V
ILX
200mA/div
2μs/div
_______________________________________________________________________________________
7
MAX8625A
Typical Operating Characteristics (continued)
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Pin Description
PIN
NAME
FUNCTION
1, 2
LX1
Inductor Connection 1. Connect the inductor between LX1 and LX2. Both LX1 pins must be connected
together externally. LX1 is internally connected to GND during shutdown.
3, 4
LX2
Inductor Connection 2. Connect the inductor between LX1 and LX2. Both LX2 pins must be connected
together externally. LX2 is internally connected to GND during shutdown.
5
ON
Enable Input. Connect ON to the input or drive high to enable the IC. Drive ON low to disable the IC.
6
SKIP
Mode Select Input. Connect SKIP to GND to enable skip mode. This mode provides the best overall
efficiency curve.
Connect SKIP to IN to enable forced-PWM mode. This mode provides the lowest noise, but reduces lightload efficiency compared to skip mode.
7
FB
Feedback Input. Connect to ground to set the fixed 3.3V output. Connect FB to the center tap of an
external resistor-divider from the output to GND to set the output voltage to a different value. VFB regulates
to 1.25V.
8
REF
Reference Output. Bypass REF to GND with a 0.1µF ceramic capacitor. VREF is 1.25V and is internally
pulled to GND during shutdown.
9, 10
OUT
Power Output. Bypass OUT to GND with two 22µF ceramic capacitors. Both OUT pins must be connected
together externally.
11, 12
GND
Ground. Connect the exposed pad and GND directly under the IC.
13, 14
IN
Power-Supply Input. Bypass IN to GND with two 22µF ceramic capacitors. Connect IN to a 2.5V to 5.5V
supply. Both IN pins must be connected together externally.
—
EP
Exposed Pad. Connect to GND directly under the IC. Connect to a large ground plane for increased
thermal performance.
Detailed Description
The MAX8625A step-up/down architecture employs a
true H-bridge topology that combines a boost converter
and a buck converter topology using a single inductor
and output capacitor (Figure 1). The MAX8625A utilizes
a pulse-width modulated (PWM), current-mode control
scheme and operates at a 1MHz fixed frequency to
minimize external component size. A proprietary
H-bridge design eliminates mode changes when transitioning from buck to boost operation. This control
scheme provides very low output ripple using a much
smaller inductor than a conventional H-bridge, while
avoiding glitches that are commonly seen during mode
transitions with competing devices.
The MAX8625A 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
MAX8625A is optimized for use in HDDs, DSCs, and
other devices requiring low-quiescent current for optimal light-load efficiency and maximum battery life.
8
Control Scheme
The MAX8625A basic noninverting step-up/down converter operates with four internal switches. The control
logic determines which two internal MOSFETs operate
to maintain the regulated output voltage. Unlike a traditional H-bridge, the MAX8625A utilizes smaller peakinductor currents, thus improving efficiency and
lowering input/output ripple.
The MAX8625A uses three operating phases during
each switching cycle. In phase 1 (fast-charge), the
inductor current ramps up with a di/dt of VIN/L. In phase
2 (slow charge/discharge), the current either ramps up
or down depending on the difference between the input
voltage and the output voltage (VIN - VOUT)/L. In phase 3
(discharge), the inductor current discharges at a rate of
VOUT/L through MOSFETs P2 and N1 (see Figure 1). An
additional fourth phase (phase 4: hold) is entered when
the inductor current falls to zero during phase 3. This
fourth phase is only used during skip operation.
The state machine (Figure 2) decides which phase to
use and when to switch phases. The converter goes
through the first three phases in the same order at all
_______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
IN
MAX8625A
LX1
LX2
P1
P2
OUT
N2
N1
UVLO
P1
CURRENT SENSE
PWM/PFM
CONTROL
ON
SKIP
OSCILLATOR
1.25V
REF
Gm
REFERENCE
125mV
GND
MAX8625
FB
Figure 1. Simplified Block Diagram
times. This reduces the ripple and removes any mode
transitions from boost-only or buck-only to hybrid modes
as seen in competing H-bridge converters.
The time spent in each phase is set by a PWM controller, using timers and/or peak-current regulation on a
cycle-by-cycle basis. The heart of the PWM control
block is a comparator that compares the output voltage-error feedback signal and the sum of the currentsense and slope compensation signals. The currentmode control logic regulates the inductor current as a
function of the output error voltage signal. The currentsense signal is monitored across the MOSFETs (P1, N1,
and N2). A fixed time delay of approximately 30ns
occurs between turning the P1 and N2 MOSFETs off,
and turning the N1 and P2 MOSFETs on. This dead
time prevents efficiency loss by preventing “shootthrough” current.
Step-Down Operation (VIN > VOUT)
During medium and heavy loads and VIN > V OUT ,
MOSFETs P1 and N2 turn on to begin phase 1 at the
clock edge and ramp up the inductor current. The
duration of phase 1 is set by an internal timer. During
phase 2, N2 turns off, and P2 turns on to further ramp
up inductor current and also transfer charge to the output. This slow charge phase is terminated on a clock
edge and P1 is turned off. The converter now enters the
fast discharge phase (phase 3). In phase 3, N1 turns
on and the inductor current ramps down to the valley
current-regulation point set by the error signal. At the
end of phase 3, both P2 and N1 turn off and another
phase 1 is initiated and the cycle repeats.
With SKIP asserted low, during light loads when inductor current falls to zero in phase 3, the converter switches to phase 4 to reduce power consumption and avoid
_______________________________________________________________________________________
9
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
FAULT
TIMEOUT
(ASYNCHRONOUS
FROM ANYWHERE)
OFF
ON = 0
P1, P2 = OFF
N1, N2 = ON
IQ = 0μA
ERROR
ON = 1
P1, P2 = OFF
N1, N2 = ON
ON = 0
(ASYNCHRONOUS
FROM
ANYWHERE)
TPUP
REFOK = 0 OR
UVLO = 0
(ASYNCHRONOUS
FROM ANYWHERE)
POWER-UP
ON = 1, P1, P2 = OFF, N1, N2 = ON,
OSC = ON AND REF = ON IF UVLO OK
TRUN
PHASE 2
SLOW CHARGE/
DISCHARGE
OSC = ON
P1, P2 = ON
N1, N2 = OFF
T1-2
PHASE 1
FAST-CHARGE
OSC = ON
P1, N2 = ON
P2, N1 = OFF
T1-3
T2-3
PHASE 3
FAST DISCHARGE
OSC = ON
P2, N1 = ON
P1, N2 = OFF
T3-1
T3-4
(SKIP)
T4-1
PHASE 4
HOLD
OSC = OFF
N1, N2 = ON
P1, P2 = OFF
Figure 2. State Diagram
shuttling current in and out of the output capacitor. If
SKIP is asserted high for forced-PWM mode, phase 4 is
not entered and current shuttling is allowed (and is
necessary to maintain the PWM operation frequency
when no load is present).
Step-Up Operation (VIN < VOUT)
During medium and heavy loads when VIN < VOUT,
MOSFETs P1 and N2 turn on at the clock edge to ramp
up the inductor current. Phase 1 terminates when the
inductor current reaches the peak target current set by
the PWM comparator and N2 turns off. This is followed
by a slow-discharge phase (phase 2) instead of a
charge phase (since VIN is less than VOUT) when P2
turns on. The slow-discharge phase terminates on a
clock edge. The converter now enters the fast-discharge phase (phase 3). During phase 3, P1 turns off
10
and N1 turns on. At the end of the minimum time, both
P2 and N1 turn off and the cycle repeats.
If SKIP is asserted low, during light loads when inductor
current falls to zero in phase 3, the converter switches to
phase 4 (hold) to reduce power consumption and avoid
shuttling current in and out of the output. If SKIP is high
to assert forced-PWM mode, the converter never enters
phase 4 and allows negative inductor current.
Step-Up/Down Transition-Zone Operation
(VIN = VOUT)
When VIN = VOUT, the converter still goes through the
three phases for moderate to heavy loads. However,
the maximum time is now spent in phase 2 where
inductor current di/dt is almost zero, since it is proportional to (VIN - VOUT). This eliminates transition glitches
______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Forced-PWM Mode
Drive SKIP high to operate the MAX8625A in forcedPWM mode. In this mode, the IC operates at a constant
1MHz switching frequency with no pulse skipping. This
scheme is desirable in noise-sensitive applications
because the output ripple is minimized and has a predictable noise spectrum. Forced PWM consumes higher
supply current at light loads due to constant switching.
Skip Mode
Drive SKIP low to operate the MAX8625A in skip mode
to improve light-load efficiency. In skip mode, the IC
switches only as necessary to maintain the output at
light loads, but still operates with fixed-frequency PWM
at medium and heavy loads. This maximizes light-load
efficiency and reduces the input quiescent current to
37µA (typ).
Load Regulation and Transient Response
During a load transient, the output voltage instantly
changes due to the ESR of the output capacitors by an
amount equal to their ESR times the change in load
current (ΔVOUT = RESR x ΔILOAD). The output voltage
then deviates further based on the speed at which the
loop compensates for the load step. Increasing the output capacitance reduces the output-voltage droop. See
the Capacitor Selection section. The typical application
circuit limits the output transient droop to less than 3%.
See the Typical Operating Characteristics section.
Soft-Start
Soft-start prevents input inrush current during startup.
Internal soft-start circuitry ramps the peak inductor current with an internal DAC in 8ms. Once the output
reaches regulation, the current limit immediately jumps
to the maximum threshold. This allows full load capability as soon as regulation is reached, even if it occurs
before the 8ms soft-start time is complete.
Shutdown
Drive ON low to place the MAX8625A in shutdown
mode and reduce supply current to less than 1µA.
During shutdown, OUT is disconnected from IN, and
LX1 and LX2 are connected to GND. Drive ON high for
normal operation.
Fault and Thermal Shutdown
The MAX8625A contains current-limit and thermal shutdown circuitry to protect the IC from fault conditions.
When the inductor current exceeds the current limit (2A
for the MAX8625A), the converter immediately enters
phase 3 and an internal 100ms timer starts. The converter continues to commutate through the three phases, spending most of its time in phase 1 and phase 3. If
the overcurrent event continues and the output is out of
regulation for the duration of the 100ms timer, the IC
enters shutdown mode and the output latches off. ON
must then be toggled to clear the fault. If the overload
is removed before the 100ms timer expires, the timer is
cleared and the converter resumes normal operation.
The thermal-shutdown circuitry disables the IC switching if the die temperature exceeds +165°C. The IC
begins soft-start once the die temperature cools by
15°C.
______________________________________________________________________________________
11
MAX8625A
or oscillation between the boost and buck modes as
seen in other step-up/down converters. See the switching waveforms for each of the three modes and transition waveforms in the Typical Operating Characteristics
section.
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Applications Information
See the Typical Operating Characteristics section for
the Maximum Load Current vs. Input Voltage graph.
Selecting the Output Voltage
Capacitor Selection
The MAX8625A output is nominally fixed at 3.3V.
Connect FB to GND to select the internally fixed-output
voltage. For an adjustable output voltage, connect FB
to the center tap of an external resistor-divider connected from the output to GND (R1 and R2 in Figure 3).
Select 100kΩ for R2 and calculate R1 using the following equation:
The input and output ripple currents are both discontinuous in this topology. Therefore, select at least two
22µF ceramic capacitors at the input. Select two 22µF
ceramic output capacitors. For best stability over a
wide temperature range, use X5R or better dielectric.
Inductor Selection
The recommended inductance range for the
MAX8625A is 3.3µH to 4.7µH. Larger values of L give a
smaller ripple, while smaller L values provide a better
transient response. This is because, for boost and stepup/down topologies, the crossover frequency is
inversely proportional to the value of L for a given load
and input voltage. The MAX8625A is internally compensated, and therefore, the choice of power components
for stable operation is bounded. A 3.3µH inductor with
2A rating is recommended for the 3.3V fixed output with
0.8A load.
⎛V
⎞
R1 = 100kΩ × ⎜ OUT − 1⎟
⎝ VFB
⎠
where VFB = 1.25V and VOUT is the desired output regulation voltage. VOUT must be between 1.25V and 4V.
Note that the minimum output voltage is limited by the
minimum duty cycle. VOUT cannot be below 1.25V.
Calculating Maximum Output Current
The maximum output current provided by the MAX8625A
circuit depends on the inductor value, switching frequency, efficiency, and input/output voltage.
L
3.3μH
1
INPUT
2.7V TO 5.5V
13
14
C1, C2
22μF
2
LX1
LX1
3
4
LX2
LX2
IN
OUT
IN
OUT
10
MAX8625A
6
C3, C4
22μF
R1
140kΩ
U1
MODE
SELECTION
INPUT
OUTPUT
3V
9
FB
7
SKIP
ON
5
OFF
8
R2
100kΩ
ON
REF
C5
0.1μF
11
GND
GND 12
Figure 3. Typical Application Circuit (Adjustable Output)
12
______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MAX8625A
L
3.3μH
1
INPUT
2.7V TO 5.5V
13
14
C1, C2
22μF
2
LX1
LX1
3
4
LX2
LX2
IN
IN
9
OUT
10
OUT
OUTPUT
3.3V
C3, C4
22μF
U1
MODE
SELECTION
INPUT
MAX8625A
6
FB
7
SKIP
ON
5
OFF
8
ON
REF
C5
0.1μF
11
GND
12
GND
Figure 4. Typical Application Circuit (Fixed 3.3V Output)
Chip Information
PROCESS: BiCMOS
______________________________________________________________________________________
13
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
6, 8, &10L, DFN THIN.EPS
MAX8625A
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
14
______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
COMMON DIMENSIONS
PACKAGE VARIATIONS
SYMBOL
MIN.
MAX.
PKG. CODE
N
D2
E2
e
JEDEC SPEC
b
[(N/2)-1] x e
A
0.70
0.80
T633-2
6
1.50±0.10
2.30±0.10
0.95 BSC
MO229 / WEEA
0.40±0.05
1.90 REF
D
2.90
3.10
T833-2
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
E
2.90
3.10
T833-3
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
A1
0.00
0.05
T1033-1
10
1.50±0.10
2.30±0.10
0.50 BSC
MO229 / WEED-3
0.25±0.05
2.00 REF
L
0.20
0.40
T1033-2
10
1.50±0.10
2.30±0.10
0.50 BSC
MO229 / WEED-3
0.25±0.05
2.00 REF
k
0.25 MIN.
T1433-1
14
1.70±0.10
2.30±0.10
0.40 BSC
----
0.20±0.05
2.40 REF
A2
0.20 REF.
T1433-2
14
1.70±0.10
2.30±0.10
0.40 BSC
----
0.20±0.05
2.40 REF
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2008 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products, Inc.
MAX8625A
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)