MAX8625A_09 [MAXIM]
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter; 高效，无缝切换，升压/降压型DC -DC转换器
| 型号： | MAX8625A_09 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter |
| 文件： | 总16页 (文件大小：547K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1006; Rev 4; 4/09
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
General Description
Features
The MAX8625A PWM step-up/down regulator is intend-
ed 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.
o Four Internal MOSFET True H-Bridge Buck/Boost
o Glitch-Free, Buck-Boost Transitions
o Minimal Output Ripple Variation on Transitions
o Up to 92% Efficiency
o 37µA (typ) Quiescent Current in Skip Mode
o 2.5V to 5.5V Input Range
o Fixed 3.3V or Adjustable Output
o 1µA (max) Logic-Controlled Shutdown
o True Shutdown
o Output Overload Protection
o Internal Compensation
o Internal Soft-Start
o 1MHz Switching Frequency
o Thermal-Overload Protection
o Small 3mm x 3mm, 14-Pin TDFN Package
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, fixed-
frequency 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 addi-
tion, 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.
Ordering Information
PIN-
PART
TOP MARK
PACKAGE
MAX8625AETD+
14 TDFN-EP**
ABQ
Applications
Note: The device is specified over the -40°C to +85°C extended
temperature range.
PDAs and Smartphones
DSCs and Camcorders
+Denotes a lead(Pb)-free/RoHS-compliant package.
**EP = Exposed pad.
MP3 Players and Cellular Phones
Battery-Powered Hard Disk Drive (HDD)
Pin Configuration
Typical Operating Circuit
TOP VIEW
14 13 12 11 10
9
8
OUTPUT
3.3V
INPUT
2.7V TO 5.5V
LX1
LX2
OUT
IN
GND
FB
MAX8625A
PWM
SKIP
MAX8625A
EP
+
SKIP
ON
1
2
3
4
5
6
7
REF
ON
TDFN-EP
OFF
EP = EXPOSED PAD.
*U.S. Patent #7,298,119.
True Shutdown is a trademark of Maxim Integrated Products, Inc.
________________________________________________________________ Maxim Integrated Products
1
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.
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
Operating Temperature Range ...........................-40°C to +85°C
IN, OUT, SKIP, ON to GND ......................................-0.3V to +6V
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
REF, FB, to GND...............................................-0.3V, (IN + 0.3V)
LX2, LX1 (Note 1)......................................................... 1.5A
RMS
Continuous Power Dissipation (T = +70°C)
A
Single-Layer Board (derate 18.5mW/°C
above T = +70°C) ...................................................1482mW
A
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.
MX8625A
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 = 3.6V, ON = SKIP = IN, FB = GND, V
= 3.3V, LX_ unconnected, C = C5 = 0.1µF to GND, Figure 4. T = -40°C to +85°C.
REF A
IN
OUT
Typical values are at T = +25°C, unless otherwise noted.) (Note 2)
A
PARAMETER
Supply Range
SYMBOL
CONDITIONS
MIN
2.5
TYP
MAX
5.5
UNITS
V
V
V
IN
UVLO Threshold
UVLO
V
rising, 60mV hysteresis
2.20
2.49
IN
Quiescent Supply Current, FPWM
Mode, Switching
I
IN
I
IN
I
IN
I
IN
No load, V
= 3.2V
15
37
35
22
mA
µA
µA
OUT
Quiescent Supply Current, Skip
Mode, Switching
SKIP = GND, no load
Quiescent Supply Current, No
Switching, Skip Mode
SKIP = GND, FB = 1.3V
45
1
ON = GND, T = +25°C
0.1
0.2
A
Shutdown Supply Current
µA
V
T
A
= +85°C
PWM mode, V = 2.5V to 5.5V
3.30
IN
I
T
= 0 to 0.5A, V = 2.5V to 5.5V,
IN
= -40°C to +85°C (Note 3)
OUT
-1
+1
%
V
A
Output Voltage Accuracy
(Fixed Output)
SKIP mode, valley regulation value
Average skip voltage
3.28
3.285
-3
Load step +0.5A
%
V
Output Voltage Range
(Adjustable Output)
1.25
4.00
Maximum Output Current
Soft-Start
V
= 3.6V
0.80
250
0.1
0.03
3
A
mA/ms
%/A
%/V
µA
IN
L = 3.3µH; C
= C3 + C4 = 44µF
OUT
Load Regulation
Line Regulation
I
= 0 to 500mA
= 2.5V to 5.5V
OUT
V
V
V
IN
OUT Bias Current
REF Output Voltage
REF Load Regulation
I
= 3.3V
OUT
OUT
V
= 2.5V to 5.5V
1.244
1.244
1.25
1
1.256
1.258
V
REF
IN
I
= 10µA
mV
REF
I
= 0 to full load, PWM mode; V = 2.5V
OUT
IN
FB Feedback Threshold
V
1.25
V
FB
to 5.5V
2
_______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
ELECTRICAL CHARACTERISTICS (continued)
(V = 3.6V, ON = SKIP = IN, FB = GND, V
= 3.3V, LX_ unconnected, C
= C5 = 0.1µF to GND, Figure 4. T = -40°C to +85°C.
REF A
IN
OUT
Typical values are at T = +25°C, unless otherwise noted.) (Note 2)
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
100
MAX
125
0.1
UNITS
FB Dual-Mode Threshold
V
75
mV
FBDM
V
V
= 1.3V, T = +25°C
0.001
0.01
FB
FB
A
FB Leakage Current
I
FB
µA
= 1.3V, T = +85°C
A
ON, SKIP Input High Voltage
ON, SKIP Input Low Voltage
V
2.5V < V < 5.5V
1.6
V
V
IH
IN
V
2.5V < V < 5.5V
0.45
1
IL
IN
2.5V < V < 5.5V, T = +25°C
0.001
0.01
3
IN
A
ON Input Leakage Current
I
µA
µA
IHL
T
A
= +85°C
I
V
V
= 3.6V
= 0V
12
SKIPH
SKIP
SKIP
SKIP Input Leakage Current
I
-2
-0.2
2000
100
0.05
SKIPL
Peak Current Limit
I
LX1 PMOS
1700
2300
mA
ms
LIMP
Fault Latch-Off Delay
Each MOSFET, T = +25°C
0.1
0.2
A
MOSFET On-Resistance
R
Ω
ON
Each MOSFET, V = 2.5V to 5.5V,
IN
T
A
= -40°C to +85°C
Rectifier-Off Current Threshold
I
SKIP = GND
125
100
300
mA
mA
LX1OFF
SKIP = GND, load decreasing
Load increasing
Idle-Mode Current Threshold
(Note 4)
I
SKIP
V
V
= V
= 5.5V, V
= 0V to V ,
IN
OUT
LX1 IN
0.01
0.2
1
1
= 0V to V
, T = +25°C
OUT A
LX1, LX2 Leakage Current
Out Reverse Current
I
µA
µA
LX2
LXLKG
T
A
= +85°C
V
= V
= V
= 0V, V
= 5.5V,
IN
LX1
LX2
OUT
0.01
measure I (LX2), T = +25°C
I
A
LXLKGR
T
A
= +85°C
0.5
25
Minimum T
T
%
kHz
°C
ON
ONMIN
OSC Frequency
F
850
1000
+165
1150
OSCPWM
Thermal Shutdown
15°C hysteresis
Note 2: Devices are production tested at T = +25°C. Specifications over the operating temperature range are guaranteed by
A
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
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Typical Operating Characteristics
(V = 3.6V, SKIP = GND, T = +25°C, Figure 4, unless otherwise noted.)
A
IN
SKIP-MODE EFFICIENCY
vs. INPUT VOLTAGE
EFFICIENCY vs. LOAD CURRENT
FPWM MODE (FIGURE 3)
EFFICIENCY vs. LOAD CURRENT
SKIP AND FPWM MODES
100
95
90
85
80
75
70
65
60
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
300mA
MX8625A
100mA
500mA
V
= 3.3V
V
= 2.8V
OUT
OUT
V
= 2.7V
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
V
= 2.7V
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
IN
IN
V
= 3.3V
OUT
LOAD CURRENT = 100mA,
300mA, 500mA
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
INPUT VOLTAGE (V)
0.1
1
10
100
1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
OUTPUT VOLTAGE (3.3V INTERNAL FB)
vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
FPWM MODE (FIGURE 3)
OUTPUT VOLTAGE (2.8V EXTERNAL FB)
vs. LOAD CURRENT (FIGURE 3)
2.0
1.5
100
90
80
70
60
50
40
30
20
10
0
2.0
1.5
1.0
0.5
0
1.0
0.5
0
V
= 3.45V
OUT
V
-0.5
-1.0
-1.5
-2.0
= 2.7V
3.0V,
3.3V,
3.6V,
4.2V,
5.0V
-0.5
-1.0
-1.5
-2.0
IN
V
T
= 3.3V
V
T
= 2.8V
OUT
A
OUT
A
= +25°C, T = -40°C, T = +85°C,
= +25°C, T = -40°C, T = +85°C
A
A
A
A
0.1
1
10
100
1000
0.1
1
10
100
1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
SUPPLY CURRENT vs. INPUT VOLTAGE
WITH NO LOAD
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH INTERNAL FB RESISTORS
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH EXTERNAL FB RESISTORS
100
10
2.82
2.81
2.80
2.79
2.78
2.77
2.76
2.75
3.33
3.32
3.31
3.30
3.29
3.28
3.27
FPWM MODE
1
0.1
0.01
NO LOAD V
= 3.3V
OUT
LOAD: 500mA, V
= 3.3V
OUT
LOAD: 500mA, V
= 2.8V
OUT
T
= +25°C, T = -40°C, T = +85°C
A
A
A
T
= +25°C, T = -40°C, T = +85°C (FIGURE 3)
A
A A
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
INPUT VOLTAGE (V)
3.0
3.5
4.0
4.5
5.0
5.5
6.0
3.0
3.5
4.0
4.5
5.0
5.5
6.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
4
_______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
Typical Operating Characteristics (continued)
(V = 3.6V, SKIP = GND, T = +25°C, Figure 4, unless otherwise noted.)
A
IN
SWITCHING WAVEFORMS
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
V
= 3V, LOAD = 500mA, V = 3.3V
IN
OUT
MAX8625A toc11
1000
900
800
700
600
500
400
300
200
100
0
V
OUT
50mV/div
(AC-COUPLED)
V
= 3.3V
OUT
V
LX1
2V/div
V
LX2
2V/div
I
LX
500mA/div
1μs/div
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
INPUT VOLTAGE (V)
SWITCHING WAVEFORMS
SWITCHING WAVEFORMS
V
= 3.3V, LOAD = 500mA, V
= 3.3V
OUT
MAX8625A toc12
V
= 3.6V, LOAD = 500mA, V
= 3.3V
OUT
MAX8625A toc13
IN
IN
V
V
OUT
OUT
50mV/div
50mV/div
(AC-COUPLED)
(AC-COUPLED)
V
V
LX1
LX1
2V/div
2V/div
V
LX2
V
LX2
2V/div
2V/div
I
LX
I
LX
500mA/div
500mA/div
1μs/div
1μs/div
FPWM MODE
= 3V, LOAD = 20mA,
SKIP MODE
= 3V, LOAD = 20mA,
V
IN
V
IN
V
= 3.308V
V
= 3.288V
OUT
OUT
MAX8625A toc15
MAX8625A toc14
V
OUT
20mV/div
(AC-COUPLED)
OUT
20mV/div
(AC-COUPLED)
V
CH1 = V
LX1
LX1
2V/div
2V/div
V
CH2 = V
LX2
LX2
2V/div
2V/div
I
LX
I
LX
500mA/div
500mA/div
1μs/div
10μs/div
_______________________________________________________________________________________
5
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Typical Operating Characteristics (continued)
(V = 3.6V, SKIP = GND, T = +25°C, Figure 4, unless otherwise noted.)
A
IN
STARTUP WAVEFORMS (FIGURE 3)
STARTUP WAVEFORMS
V
= 3.6V, LOAD = 30Ω, V
= 1.5V
IN
OUT
V
IN
= 3.6V, LOAD = 5Ω, V
= 3.288V
OUT
MAX8625A toc16
MAX8625A toc17
SHDN
2V/div
SHDN
2V/div
V
V
OUT
OUT
MX8625A
500mA/div
20mV/div
I
LX
500mA/div
I
LX
500mA/div
I
I
BATT
BATT
100mA/div
500mA/div
2ms/div
2ms/div
LINE TRANSIENT
LOAD TRANSIENT
= 3.3V
V
= 3.3V, LOAD = 5.5Ω,
OUT
V
V
RAMP 3V TO 4V
OUT
IN
MAX8625A toc18
MAX8625A toc19
V
OUT
100mV/div
(DC OFFSET = 3.3V)
CH2 = V
50mV/div
(AC-COUPLED)
OUT
I
BATT
250mA/div
CH1 = V
500mV/div
3V OFFSET
IN
I
LX
500mA/div
400μs/div
1ms/div
BODE PLOT
GAIN AND PHASE vs. FREQUENCY
OSCILLATOR FREQUENCY
vs. TEMPERATURE
MAX8625A toc20
1.06
1.04
1.02
1.00
0.98
0.96
0.94
0.92
0.90
40
30
180
144
108
72
GAIN
20
10
0
36
PHASE
= 3.6
-10
-20
-30
-40
-50
-60
0
-36
-72
-108
-144
-180
V
V
IN
= 3.3V
OUT
LOAD = 200mA
-40 -20
0
20
40
60
80 100
1
10
100
1000
TEMPERATURE (°C)
FREQUENCY (kHz)
6
_______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
Typical Operating Characteristics (continued)
(V = 3.6V, SKIP = GND, T = +25°C, Figure 4, unless otherwise noted.)
A
IN
MINIMUM STARTUP VOLTAGE
vs. TEMPERATURE
REFERENCE vs. TEMPERATURE
NO LOAD
2.48
2.46
2.44
2.42
2.40
2.38
2.36
2.34
2.32
2.30
2.28
1.28
1.27
1.26
1.25
1.24
1.23
1.22
V
= 3.3V, NO LOAD
OUT
V
V
= 3.3V
= 3.0V,
3.6V,
OUT
IN
4.2V,
5.0V
-50
-25
0
25
50
75
100
-40 -20
0
20
40
60
80 100
TEMPERATURE (°C)
TEMPERATURE (°C)
SHUTDOWN DUE TO OVERLOAD
= 3.6V, V = 3.288V
REFERENCE vs. TEMPERATURE
WITH 300mA LOAD
V
IN
OUT
MAX8625A toc25
1.28
1.27
1.26
1.25
1.24
1.23
1.22
V
LX2
2V/div
V
LX2
2V/div
V
OUT
V
V
= 3.3V
OUT
IN
500mV/div
= 3.0V,
3.6V,
4.2V,
5.0V
I
LX
500mA/div
100μs/div
-40 -20
0
20
40
60
80 100
TEMPERATURE (°C)
BOOST-TO-BUCK TRANSITION
FPWM MODE V = 3.6V, V
= 3.288V
IN
OUT
MAX8625A toc26
V
OUT
100mV/div
AC-COUPLED
V
IN
1V/div
DC OFFSET = 3V
I
LX
200mA/div
2μs/div
_______________________________________________________________________________________
7
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Pin Description
PIN
NAME
FUNCTION
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.
1, 2
LX1
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.
3, 4
5
LX2
ON
Enable Input. Connect ON to the input or drive high to enable the IC. Drive ON low to disable the IC.
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 light-
load efficiency compared to skip mode.
MX8625A
6
SKIP
Feedback Input. Connect to ground to set the fixed 3.3V output. Connect FB to the center tap of an
7
8
FB
external resistor-divider from the output to GND to set the output voltage to a different value. V regulates
FB
to 1.25V.
Reference Output. Bypass REF to GND with a 0.1µF ceramic capacitor. V
pulled to GND during shutdown.
is 1.25V and is internally
REF
REF
Power Output. Bypass OUT to GND with two 22µF ceramic capacitors. Both OUT pins must be connected
together externally.
9, 10
11, 12
13, 14
OUT
GND
IN
Ground. Connect the exposed pad and GND directly under the IC.
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.
Exposed Pad. Connect to GND directly under the IC. Connect to a large ground plane for increased
thermal performance.
—
EP
Control Scheme
The MAX8625A basic noninverting step-up/down con-
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 transi-
tioning 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.
verter operates with four internal switches. The control
logic determines which two internal MOSFETs operate
to maintain the regulated output voltage. Unlike a tradi-
tional H-bridge, the MAX8625A utilizes smaller peak-
inductor 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 V /L. In phase
IN
2 (slow charge/discharge), the current either ramps up
or down depending on the difference between the input
voltage and the output voltage (V - V
)/L. In phase 3
OUT
IN
(discharge), the inductor current discharges at a rate of
/L through MOSFETs P2 and N1 (see Figure 1). An
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 opti-
mal light-load efficiency and maximum battery life.
V
OUT
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
8
_______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
LX1
LX2
P1
P2
IN
OUT
N2
N1
UVLO
P1
CURRENT SENSE
PWM/PFM
CONTROL
ON
SKIP
OSCILLATOR
1.25V
REF
Gm
REFERENCE
125mV
GND
FB
MAX8625A
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.
Step-Down Operation (V > V
)
OUT
IN
During medium and heavy loads and V > V
,
OUT
IN
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 out-
put. 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.
The time spent in each phase is set by a PWM con-
troller, 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 volt-
age-error feedback signal and the sum of the current-
sense and slope compensation signals. The current-
mode control logic regulates the inductor current as a
function of the output error voltage signal. The current-
sense 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 “shoot-
through” current.
With SKIP asserted low, during light loads when induc-
tor current falls to zero in phase 3, the converter switch-
es to phase 4 to reduce power consumption and avoid
_______________________________________________________________________________________
9
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
OFF
ON = 0
P1, P2 = OFF
N1, N2 = ON
ERROR
ON = 0
(ASYNCHRONOUS
FROM
FAULT
ON = 1
TIMEOUT
P1, P2 = OFF
(ASYNCHRONOUS
FROM ANYWHERE)
N1, N2 = ON
ANYWHERE)
I
Q
= 0μA
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
MX8625A
TRUN
PHASE 2
SLOW CHARGE/
DISCHARGE
OSC = ON
P1, P2 = ON
N1, N2 = OFF
PHASE 1
FAST-CHARGE
OSC = ON
P1, N2 = ON
P2, N1 = OFF
T1-2
T1-3
T3-1
T2-3
T4-1
PHASE 3
FAST DISCHARGE
OSC = ON
P2, N1 = ON
P1, N2 = OFF
PHASE 4
HOLD
OSC = OFF
N1, N2 = ON
P1, P2 = OFF
T3-4
(SKIP)
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).
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 Operation (V < V
)
IN
OUT
During medium and heavy loads when V < V
IN
OUT,
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
Step-Up/Down Transition-Zone Operation
(V = V
)
IN
OUT
When V = V
, the converter still goes through the
IN
OUT
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 propor-
charge phase (since V is less than V
) when P2
OUT
IN
turns on. The slow-discharge phase terminates on a
clock edge. The converter now enters the fast-dis-
charge phase (phase 3). During phase 3, P1 turns off
tional to (V - V
). This eliminates transition glitches
OUT
IN
10 ______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
or oscillation between the boost and buck modes as
Soft-Start
Soft-start prevents input inrush current during startup.
Internal soft-start circuitry ramps the peak inductor cur-
rent with an internal DAC in 8ms. Once the output
reaches regulation, the current limit immediately jumps
to the maximum threshold. This allows full load capabil-
ity as soon as regulation is reached, even if it occurs
before the 8ms soft-start time is complete.
seen in other step-up/down converters. See the switch-
ing waveforms for each of the three modes and transi-
tion waveforms in the Typical Operating Characteristics
section.
Forced-PWM Mode
Drive SKIP high to operate the MAX8625A in forced-
PWM 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 pre-
dictable noise spectrum. Forced PWM consumes higher
supply current at light loads due to constant switching.
When using the MAX8625A at low input voltages (close
to UVLO and < 3V), it is recommended that the ON pin
should not be tied to the BATT or supply voltage node
directly. The ON pin should be held low for > 1ms after
power to the MAX8625A is applied before it is driven
high for normal operation.
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).
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 shut-
down 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 con-
verter continues to commutate through the three phas-
es, 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.
Do not dynamically transition between skip and FPWM.
The MAX8625A is not designed for dynamic transitions
between FPWM and skip modes. Spikes of negative
inductor current are possible when making these types
of dynamic transitions. The magnitude of the spike
depends on the load and output capacitance. The
MAX8625A has no protection against these types of
negative current spikes.
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 (ΔV
= R
x ΔI
). The output voltage
LOAD
OUT
ESR
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.
then deviates further based on the speed at which the
loop compensates for the load step. Increasing the out-
put 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.
______________________________________________________________________________________ 11
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
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.
Applications Information
Selecting the Output Voltage
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 connect-
ed from the output to GND (R1 and R2 in Figure 3).
Select 100kΩ for R2 and calculate R1 using the follow-
ing equation:
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 step-
up/down topologies, the crossover frequency is
inversely proportional to the value of L for a given load
and input voltage. The MAX8625A is internally compen-
sated, 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.
MX8625A
⎛
⎞
V
V
OUT
R1 = 100kΩ ×
−1
⎟
⎜
⎝
⎠
FB
where V = 1.25V and V
is the desired output reg-
FB
OUT
ulation voltage. V
must be between 1.25V and 4V.
OUT
PCB Layout and Routing
Good PCB layout is important to achieve optimal per-
formance from the MAX8625A. 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 0.2 inches 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 MAX8625A
evaluation kit for a PCB layout example.
Note that the minimum output voltage is limited by the
minimum duty cycle. V
cannot be below 1.25V.
OUT
Calculating Maximum Output Current
The maximum output current provided by the MAX8625A
circuit depends on the inductor value, switching frequen-
cy, efficiency, and input/output voltage.
See the Typical Operating Characteristics section for
the Maximum Load Current vs. Input Voltage graph.
Capacitor Selection
The input and output ripple currents are both discontin-
uous in this topology. Therefore, select at least two
L
3.3μH
1
2
3
4
INPUT
2.7V TO 5.5V
OUTPUT
3V
LX1
LX1
LX2
LX2
13
14
9
IN
IN
OUT
OUT
10
C1, C2
22μF
C3, C4
22μF
R1
140kΩ
U1
7
FB
MODE
SELECTION
INPUT
6
5
MAX8625A
SKIP
ON
R2
100kΩ
ON
OFF
11
12
8
REF
GND
GND
C5
0.1μF
Figure 3. Typical Application Circuit (Adjustable Output)
12 ______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
L
3.3μH
1
2
3
4
INPUT
2.7V TO 5.5V
OUTPUT
3.3V
LX1
LX1
LX2
LX2
13
14
9
IN
IN
OUT
OUT
10
C1, C2
22μF
C3, C4
22μF
U1
MAX8625A
7
FB
MODE
SELECTION
INPUT
6
5
SKIP
ON
ON
OFF
11
12
8
REF
GND
GND
C5
0.1μF
Figure 4. Typical Application Circuit (Fixed 3.3V Output)
Package Information
Chip Information
For the latest package outline information and land patterns, go
PROCESS: BiCMOS
to www.maxim-ic.com/packages.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
14 TDFN-EP
T1433-2
21-0137
______________________________________________________________________________________ 13
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
MX8625A
14 ______________________________________________________________________________________
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
MX8625A
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
PACKAGE VARIATIONS
COMMON DIMENSIONS
SYMBOL MIN. MAX.
PKG. CODE
T633-2
N
6
8
8
D2
1.50 0.10 2.30 0.10 0.95 BSC
1.50 0.10 2.30 0.10
1.50 0.10 2.30 0.10 0.65 BSC
b
E2
e
JEDEC SPEC
MO229 / WEEA
MO229 / WEEC
MO229 / WEEC
MO229 / WEED-3
[(N/2)-1] x e
0.40 0.05 1.90 REF
1.95 REF
0.30 0.05 1.95 REF
A
D
0.70
2.90
2.90
0.00
0.20
0.80
3.10
3.10
0.05
0.40
T833-2
0.65 BSC
0.30 0.05
E
T833-3
T1033-1
T1033MK-1
T1033-2
T1433-1
T1433-2
T1433-3F
10 1.50 0.10 2.30 0.10 0.50 BSC
0.25 0.05
2.00 REF
2.00 REF
2.00 REF
2.40 REF
2.40 REF
A1
L
10 1.50 0.10 2.30 0.10 0.50 BSC MO229 / WEED-3 0.25 0.05
10 1.50 0.10 2.30 0.10 0.50 BSC MO229 / WEED-3 0.25 0.05
k
0.25 MIN.
0.20 REF.
14 1.70 0.10 2.30 0.10 0.40 BSC
14 1.70 0.10 2.30 0.10 0.40 BSC
14 1.70 0.10 2.30 0.10 0.40 BSC
- - - -
- - - -
- - - -
0.20 0.05
0.20 0.05
A2
0.20 0.05 2.40 REF
______________________________________________________________________________________ 15
High-Efficiency, Seamless Transition,
Step-Up/Down DC-DC Converter
Revision History
REVISION
NUMBER
REVISION
DATE
PAGES
CHANGED
DESCRIPTION
0
1
2
3
4
3/08
5/08
Initial release
Added PCB Layout and Routing section
—
12
10/08
12/08
4/09
Updated Electrical Characteristics, Skip Mode and Soft-Start sections
Corrected P1 and P2 symbols in Figure 1
Corrected U.S. Patent #
2, 11
9
1
MX8625A
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.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2009 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.
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