Maxim MAX1774EMJ Dual, high-efficiency, step-down converter with backup battery switchover Datasheet

19-1810; Rev 1; 1/02
KIT
ATION
EVALU
E
L
B
A
IL
AVA
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
Features
The MAX1774 is a complete power-supply solution for
PDAs and other hand-held devices. It integrates two
high-efficiency step-down converters, a boost converter
for backup battery regulation, and four voltage detectors in a small 32-pin QFN or 28-pin QSOP package.
The MAX1774 accepts inputs from +2.7V to +28V and
provides an adjustable main output from 1.25V to 5.5V
at over 2A. The secondary core converter delivers an
adjustable voltage from 1V to 5V and can deliver up to
1.5A. Both the main and core regulators have separate
shutdown inputs.
When the AC adapter power is removed, an external Pchannel MOSFET switches input to the main battery.
When the main battery is low, the backup step-up converter sustains the main output voltage. When the backup battery can no longer deliver the required load, the
system shuts down safely to prevent damage. Four onboard voltage detectors monitor the status of the AC
adapter power, main battery, and backup battery.
The MAX1774 evaluation kit is available to help reduce
design time.
♦ Dual, High-Efficiency, Synchronous-Rectified
Step-Down Converters
________________________Applications
♦ Input Voltage Range from +2.7V to +28V
♦ Thin, Small (1mm High) QFN Package
♦ Step-Up Converter for Backup Battery
♦ Main Power
Adjustable from +1.25V to +5.5V
Over 2A Load Current
Up to 95% Efficiency
♦ Core Power
Adjustable from 1V to 5V
Internal Switches
Up to 1.5A Load Current
Up to 91% Efficiency
♦ Automatic Main Battery Switchover
♦ 100% (max) Duty Cycle
♦ Up to 1.25MHz Switching Frequency
♦ Four Low-Voltage Detectors
Hand-Held Computers
PDAs
Internet Access Tablets
POS Terminals
Subnotebooks
♦ 170µA Quiescent Current
♦ 8µA Shutdown Current
♦ Digital Soft-Start
♦ Independent Shutdown Inputs
LBO
INS
N.C.
GND
25
26
27
LXC
28
SHDNM
N.C.
29
30
BKUP
32
GND
31
TOP VIEW
SHDNC
Pin Configurations
MDRV
1
24
ACO
PGNDC
2
23
INC
PGND
3
22
GND
NDRV
4
21
FBC
CVL
5
20
CS-
IN
6
19
CS+
PDRV
7
18
FBM
CVH
8
17
N.C.
MAX1774
Ordering Information
PART
MAX1774EEI
MAX1774EMJ
11
12
13
14
15
16
BIN
BKOFF
ACI
DBI
LBI
REF
9
10
LXB
LXB2
32 7mm x 7mm QFN
PIN-PACKAGE
28 QSOP
32 7mm x 7mm QFN
Functional Diagram
AC
ADAPTER
MAIN (+3.3V)
MAIN
BATTERY
MAX1774
CORE (+1.8V)
AC OK
LOW MAIN BATTERY
BACKUP
BATTERY
GND
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
DEAD MAIN BATTERY
GND
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX1774
General Description
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
ABSOLUTE MAXIMUM RATINGS
IN, SHDNM, MDRV, DBI, LBI, ACI,
CVH to GND .......................................................-0.3V to +30V
IN to CVH, PDRV ......................................................-0.3V to +6V
BIN to CS-.................................................................-0.3V to +6V
LXB to GND ................................................-0.3V to (VBIN+ 0.7V)
PDRV to GND..................................(VCVH - 0.3V) to (VIN + 0.3V)
All Other Pins to GND...............................................-0.3V to +6V
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation
28-Pin QSOP (derate 10.8mW/°C above +70°C)........860mW
32-Pin QFN (derate 23.2mW/°C above +70°C) ........1860mW
Operating Temperature .......................................-40°C to +85°C
Storage Temperature.........................................-65°C to +150°C
Temperature (soldering, 10s) ..........................................+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
(Figure 1, VIN = VINS +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at
TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MAX
UNITS
28
V
18
40
µA
VFBM = +1.5V, VFBC = +1.5V,
V SHDNM = V SHDNC = +3.3V
110
220
µA
IINC
VFBM = +1.5V, VFBC = +1.5V,
V SHDNM = V SHDNC = +3.3V
60
105
µA
IBIN
VBIN = +3.3V, CS- open
VFBM = +1.5V, V SHDNM = +3.3V,
V BKOFF = +1.5V, SHDNC = GND
60
105
µA
SHDNM = SHDNC = GND
8
40
µA
5.5
V
1.25
1.29
V
Input Voltage
VIN
Input Quiescent Supply Current
IIN
VFBM = +1.5V, VFBC = +1.5V,
V SHDNM = V SHDNC = +3.3V
CS- Quiescent Supply Current
ICS-
Core Regulator Quiescent
Supply Current
Backup Mode BIN Quiescent
Supply Current
IN Shutdown Supply Current
MIN
TYP
2.7
MAIN REGULATOR
Main Output Voltage Adjust
Range
1.25
FBM Regulation Threshold
VFBM
FBM Input Current
IFBM
V(CS+ - CS-) = 0 to +60mV,
VIN = +3.5V to +28V
1.21
VFBM = +1.3V
-0.1
0.1
µA
Current-Limit Threshold
VCS+ - VCS-
60
80
100
mV
Minimum Current-Limit
Threshold
VCS+ - VCS-
5
15
25
mV
Valley Current Threshold
VCS+ - VCS-
40
50
60
mV
Zero Current Threshold
VCS+ - VCS-
0
5
15
mV
PDRV, NDRV Gate Drive
Resistance
VCS- = +3.3V, IPDRV, INDRV = 50mA
2
5.5
Ω
CS- to CVL Switch Resistance
ICVL = 50mA
4.5
9.5
PDRV, NDRV Dead Time
2
50
_______________________________________________________________________________________
Ω
ns
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
(Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are
at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Maximum Duty Cycle
100
%
Minimum On-Time
200
400
650
ns
Minimum Off-Time
200
400
650
ns
5.5
V
CORE REGULATOR
Input Voltage Range
VINC
INC Undervoltage Lockout
2.6
VINC rising
2.40
2.47
2.55
VINC falling
2.30
2.37
2.45
Core Output Voltage Adjust Range
1.0
Maximum Core Load Current
VCORE = 1.8V (Note 1)
FBC Regulation Threshold
VFBC
FBC Input Current
IFBC
Dropout Voltage
ILXC
1.5
VINC = +2.5 to +5.5V, I O U T C = 0 to 200mA
0.97
1.0
1.03
V
VFBC = +1.3V
-0.1
0.1
µA
0.1
0.25
V
10
µA
0.25
0.5
Ω
VINC = +5.5V, V L X C = 0 to +5.5V
-10
LXC P-Channel, N-Channel OnResistance
LXC P-Channel Current Limit
V
1
IOUTC = 400mA
LXC Leakage Current
5.0
V
ICLC
A
1200
1800
3000
mA
LXC P-Channel Minimum Current
100
250
400
mA
LXC N-Channel Valley Current
900
1400
2400
mA
LXC N-Channel Zero-Crossing
Current
40
110
170
mA
LXC Dead Time
50
ns
Max Duty Cycle
100
Minimum On-Time
170
400
690
ns
%
Minimum Off-Time
170
400
690
ns
5.5
V
BACKUP REGULATOR
Backup Battery Input Voltage
VBBATT
LXB N-Channel On-Resistance
0.9
1.9
3.5
Ω
350
600
mA
VLXB = +5.5V, VFBM = +1.3V
1
µA
VBIN = +5.5V, CS- = BKOFF =
SHDNC = SHDNM = GND
1
µA
VCS- = +3.3V, ILXB = 50mA
LXB Current Limit
200
LXB Leakage Current
BIN Leakage Current
IBIN
BIN, CS- Switch Resistance
VCS- = +3.3V, BKOFF = GND,
SHDNM = CVL
7.5
15
Ω
BIN Switch Zero-Crossing
Threshold
VBIN = +2.5V, BKOFF = SHDNC =
SHDNM = CVL
17
35
mV
5.6
9.2
µs
LXB Maximum On-Time
Zero Crossing Detector Timeout
2.8
40
µs
_______________________________________________________________________________________
3
MAX1774
ELECTRICAL CHARACTERISTICS (continued)
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are
at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
1.23
1.25
UNITS
REFERENCE
Reference Voltage
1.27
V
Reference Load Regulation
VREF
IREF = 0 to 50µA
10
mV
Reference Line Regulation
VCS- = +2.5V to +5.5V, IREF = 50µA
5
mV
Reference Sink Current
10
µA
CVL, CVH REGULATORS
CVL Output Voltage
VCVL
CVL Switchover Threshold
CVH Output Voltage
CVH Switchover Threshold
ICVL = 50mA, VCS- = 0
2.6
ICVL = 50mA, VCS- = +3.3V
CS- rising, hysteresis = 100mV typical
CVL Undervoltage Lockout
3.1
3.2
2.40
2.47
2.55
VIN = +4V, ICVH = 25mA
VIN 3.4
VIN 2.8
VIN = +12V, ICVH = 50mA
VIN 4.2
VIN 3.7
V
V
V
VCVH
VIN
2.8
VIN rising, hysteresis = 350mV typ
5.5
V
VCVL rising
2.40
2.47
2.55
VCVL falling
2.30
2.37
2.45
V BKOFF rising
0.51
0.55
0.59
V BKOFF falling
0.46
0.50
0.54
V
LOW-VOLTAGE COMPARATORS
Backup Regulator Shutdown
Threshold
V BKOFF
BKOFF Input Bias Current
V BKOFF = +5.5V
V
1
µA
LBI Threshold
VLBI
VLBI falling, hysteresis = 50mV typical
1.17
1.20
1.23
V
DBI Threshold
VDBI
VDBI falling, hysteresis = 50mV typical
1.17
1.20
1.23
V
BKUP Low-Input Threshold
0.4
V
LBI, DBI Input Leakage Current
VLBI = VDBI = +1.3V
100
nA
LBO, BKUP, ACO, MDRV
Output Low
ISINK = 1mA
0.4
V
LBO, BKUP, ACO, MDRV
Output Leakage Current
VLBI = +1.3V, VACI = +12V, V ACO =
V LBO = V BKUP = +5.5V, V MDRV = +28V
1.0
µA
ACI Threshold
VACI – VINS, ACI falling
0.35
V
ACI Input Leakage Current
VACI = +1.3V
100
nA
INS Input Leakage Current
VINS = +3.3V
10
µA
0.4
V
0.22
1.5
LOGIC INPUTS
SHDNM, SHDNC Input Low
Voltage
SHDNM, SHDNC Input High
Voltage
4
2.0
_______________________________________________________________________________________
V
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
(Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are
at TA = +25°C.)
PARAMETER
SYMBOL
SHDNM, SHDNC Input Low
Current
CONDITIONS
SHDNM = SHDNC = GND
SHDNC Input High Current
V SHDNC = +5.5V
SHDNM Input High Current
V SHDNM = +5V
MIN
TYP
-1
2
MAX
UNITS
1
µA
5
µA
25
µA
ELECTRICAL CHARACTERISTICS
(Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
2.7
28
V
Input Voltage
VIN
Input Quiescent Supply Current
IIN
VFBM = +1.5V, VFBC = +1.5V, V SHDNM =
V SHDNC = +3.3V
40
µA
CS- Quiescent Supply Current
ICS-
VFBM = +1.5V, VFBC = +1.5V, V SHDNM =
V SHDNC = +3.3V
220
µA
Core Regulator Quiescent
Supply Current
IINC
VFBM = +1.5V, VFBC = +1.5V, V SHDNM =
V SHDNC = +3.3V
105
µA
Backup Mode BIN Quiescent
Supply Current
IBIN
VBIN = +3.3V, CS- open
VFBM = +1.5V, V SHDNM = +3.3V,
V BKOFF = +1.5V, SHDNC = GND
110
µA
SHDNM = SHDNC = GND
40
µA
1.25
5.5
V
IN Shutdown Supply Current
MAIN REGULATOR
Main Output Voltage Adjust
Range
FBM Regulation Threshold
VFBM
V(CS+ - CS-) = 0 to +60mV,
VIN = +3.5V to +28V
1.21
1.29
V
FBM Input Current
IFBM
VFBM = +1.3V
-0.1
0.1
µA
Current-Limit Threshold
VCS+ - VCS-
60
100
mV
Minimum Current-Limit
Threshold
VCS+ - VCS-
5
25
mV
Valley Current Threshold
VCS+ - VCS-
40
60
mV
Zero Current Threshold
VCS+ - VCS-
0
15
mV
PDRV, NDRV Gate Drive
Resistance
VCS- = +3.3V, IPDRV, INDRV = 50mA
5.5
Ω
CS- to CVL Switch Resistance
ICVL = 50mA
9.5
Ω
Maximum Duty Cycle
100
%
Minimum On-Time
200
650
ns
Minimum Off-Time
200
650
ns
_______________________________________________________________________________________
5
MAX1774
ELECTRICAL CHARACTERISTICS (continued)
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN = VINS = +12V, VINC = VCS+ = VCS- = +3.3V, VCORE = +1.8V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
V
CORE REGULATOR
Input Voltage Range
VINC
INC Undervoltage Lockout
2.6
5.5
VINC rising
2.39
2.55
VINC falling
2.29
2.45
1.0
5.0
Core Output Voltage Adjust
Range
Maximum Core Load Current
FBC Regulation Threshold
VFBC
FBC Input Current
IFBC
Dropout Voltage
LXC Leakage Current
VCORE = 1.8V (Note 1)
1
VINC = +2.5 to +5.5V,
IOUTC = 0 to 200mA
0.97
VFBC = +1.3V
-0.1
IOUTC = 400mA
ILXC
VINC = +5.5V, VLXC = 0 to +5.5V
-10
LXC P-Channel, N-Channel
On-Resistance
V
V
A
1.03
V
0.1
µA
0.25
V
10
µA
0.5
Ω
LXC P-Channel Current Limit
1200
3010
mA
LXC P-Channel Minimum
Current
100
420
mA
LXC N-Channel Valley Current
880
2450
mA
LXC N-Channel Zero-Crossing
Current
40
170
mA
Max Duty Cycle
100
Minimum On-Time
160
700
ns
Minimum Off-Time
170
690
ns
0.9
5.5
V
3.5
Ω
600
mA
%
BACKUP REGULATOR
Backup Battery Input Voltage
VBBATT
LXB N-Channel On Resistance
VCS- = +3.3V, ILXB = 50mA
LXB Current Limit
200
LXB Leakage Current
VLXB = +5.5V, VFBM = +1.3V
1
µA
BIN Leakage Current
VBIN = +5.5V, CS- = BKOFF =
SHDNC = SHDNM = GND
1
µA
BIN, CS- Switch Resistance
VCS- = +3.3V, BKOFF = GND,
SHDNC = CVL
15
Ω
BIN Switch Zero-Crossing
Threshold
VBIN = +2.5V, BKOFF = SHDNC =
SHDNM = CVL
35
mV
2.8
9.2
µs
1.220
IBIN
LXB Maximum On-Time
REFERENCE
Reference Voltage
1.275
V
Reference Load Regulation
IREF = 0 to 50µA
10
mV
Reference Line Regulation
VCS- = +2.5V to +5.5V, IREF = 50µA
5
mV
Reference Sink Current
6
VREF
10
_______________________________________________________________________________________
µA
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
(Figure 1, VIN = VINS = +12V, VINC = VCS+ = VCS- = +3.3V, VCORE = +1.8V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
ICVL = 50mA, VCS- = 0
2.6
3.1
V
VCS- rising, hysteresis = 100mV typical
2.40
2.55
V
CVL, CVH REGULATORS
CVL Output Voltage
VCVL
CVL Switchover Threshold
VIN = +4V, ICVH = 25mA
CVH Output Voltage
VIN - 2.8
VCVH
V
VIN = +12V, ICVH = 50mA
CVL Undervoltage Lockout
VIN - 3.65
VCVL rising
2.40
2.57
VCVL falling
2.30
2.47
V BKOFF rising
0.51
0.59
V BKOFF falling
0.46
0.54
V
LOW-VOLTAGE COMPARATORS
Backup Regulator Shutdown
Threshold
V BKOFF
BKOFF Input Bias Current
V BKOFF = +5.5V
V
1
µA
LBI Threshold
VLBI
VLBI falling, hysteresis = 50mV typical
1.17
1.23
V
DBI Threshold
VDBI
VDBI falling, hysteresis = 50mV typical
1.17
1.23
V
BKUP Low-Input Threshold
0.4
V
LBI, DBI Input Leakage Current
VLBI, VDBI = +28V
100
nA
LBO, BKUP, ACO, MDRV
Output Low
ISINK = 1mA
0.4
V
LBO, BKUP, ACO, MDRV
Output Leakage Current
VLBI = +1.3V, VACI = VIN = +12V, VACO =
V LBO = V BKUP = +5.5V, V MDRV = +28V
1.0
µA
ACI Threshold
VACI - VINS, ACI falling
0.5
V
ACI Input Leakage Current
VACI = +1.3V
100
nA
MAIN Input Leakage Current
LOGIC INPUTS
VINS = +3.3V
10
µA
0.4
V
SHDNM, SHDNC Input Low
Voltage
SHDNM, SHDNC Input High
Voltage
SHDNM, SHDNC Input Low
Current
2.0
SHDNM = SHDNC = GND
-1
V
1
µA
SHDNC Input High Current
V SHDNC = +5.5V
5
µA
SHDNM Input High Current
V SHDNM = +28V
25
µA
Note 1: This parameter is guaranteed based on the LXC P-channel current limit and the LXC N-channel valley current.
Note 2: Specifications to -40°C are guaranteed by design and not production tested.
_______________________________________________________________________________________
7
MAX1774
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(Circuit of Figure 1, VIN = +5V, VINC = +3.3V, TA = +25°C, unless otherwise noted.)
CORE EFFICIENCY vs. LOAD
90
VIN = +12V
60
VIN = +15V
50
40
VIN = +18V
30
20
70
VIN = +2.7V
60
VIN = +3.3V
50
VIN = +5V
40
30
70
60
50
1
10
100
LOAD (mA)
1000
10,000
0
1
10
100
1000
0.01
0.1
LOAD (mA)
1
10
100
LOAD (mA)
VREF ACCURACY vs. TEMPERATURE
REFERENCE LOAD REGULATION
1.5
MAX1774-05
0
MAX1774-04
2.0
-0.2
-0.4
1.0
VREF ACCURACY (%)
VREF ACCURACY (%)
VMAIN = 3.3V
10
0
0
VBBATT = +0.8V
40
20
10
10
VBBATT = +1.0V
30
VCORE = 1.8V
20
VMAIN = 3.3V
VBBATT = +2.5V
80
EFFICIENCY (%)
EFFICIENCY (%)
70
90
80
VIN = +3.3V
VIN = +5V
80
100
MAX1774-02
MAX1774-01
90
BACKUP EFFICIENCY vs. LOAD
100
0.5
0
-0.5
-1.0
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.5
-2.0
-40
-1.8
-2.0
-20
0
20
60
40
TEMPERATURE (°C)
80
100
0
10
20
30
40
50
60
70
80
IREF (µA)
MAIN SWITCHING WAVEFORMS
(LIGHT LOAD 100mA)
MAX1774-06
MAIN SWITCHING WAVEFORMS
(HEAVY LOAD 1A)
5V
MAX1774-07
LX
5V/div
4V
LX
5V/div
0
0
40mV
20mV
0
20mV
VMAIN
(AC-COUPLED)
20mV/div
8
VMAIN
(AC-COUPLED)
20mV/div
0
-20mV
-20mV
1500mA
500mA ILI
500mA/div
0
1000mA IL1
500mA/div
500mA
0
5µs/div
MAX1774-03
MAIN EFFICIENCY vs. LOAD
100
EFFICIENCY (%)
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
5µs/div
_______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
CORE SWITCHING WAVEFORMS
(HEAVY LOAD 500mA)
CORE SWITCHING WAVEFORMS
(LIGHT LOAD 50mA)
MAX1774-09
MAX1774-08
3.3V
LX
2V/div
2V
LXC
0
0
500mA
0
4V
2V/div
20mV
IL2
500mA/div
0
-20mV
VCORE
(AC-COUPLED)
20mV/div
500mA
0
VCORE
(AC-COUPLED)
20mV/div
L2
500mA/div
2µs/div
1µs/div
CORE LINE-TRANSIENT RESPONSE
MAIN LINE-TRANSIENT RESPONSE
MAX1774-11
MAX1774-10
4V
12V
10V
VINC
2V 2V/div
VIN
5V/div
5V
0
0
50mV
VCORE
(AC-COUPLED)
50mV/div
VMAIN
(AC-COUPLED)
50mV/div
-50mV
100µs/div
1µs/div
MAIN LOAD-TRANSIENT RESPONSE
MAIN LOAD-TRANSIENT RESPONSE
50mA TO 500mA
MAX1774-12
MAX1774-13
1000mA
500mA
0
500mA
IMAIN
500mA/div
0
IMAIN
500mA/div
20mV
VMAIN
(AC-COUPLED)
-20mV 20mV/div
10mV
0
0
VMAIN
(AC-COUPLED)
10mV/div
-10mV
100µs/div
100µs/div
_______________________________________________________________________________________
9
MAX1774
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +5V, VINC = +3.3V, TA = +25°C, unless otherwise noted.)
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +5V, VINC = +3.3V, TA = +25°C, unless otherwise noted.)
BACKUP SWITCHOVER RESPONSE
TURN-ON RESPONSE
MAX1774-15
MAX1774-14
5V
VSHDN
5V/div 0
3V
VMAIN
VCORE
2V
VBKUP
5V/div
VOUT
1V/div
IBBATT
50mA/div
1V
0
400µA
INPUT CURRENT
200µA
VBIN
10mV/div
INPUT
CURRENT
200mA/div
VMAIN
10mV/div
0
100µs/div
5µs/div
Pin Description
PIN
NAME
FUNCTION
QSOP
QFN
1
30
SHDNM
Shutdown for Main Regulator. Low voltage on SHDNM shuts off the main output. For normal
operation, connect SHDNM to IN.
2
31
SHDNC
Shutdown for Core Regulator. Low voltage on SHDNC shuts off the core output. For normal
operation, connect SHDNC to CVL.
3
32
BKUP
Open-Drain Backup Input/Output. The device is in backup mode when BKUP is low. BKUP can be
externally pulled low to place the device in backup mode.
4
1
MDRV
Open-Drain Drive Output. MDRV goes low when the ACI voltage drops below the main voltage plus
220mV and device is not in backup. Connect MDRV to the gate of the main battery P-channel
MOSFET to switch the battery to IN when the AC adapter voltage is not present.
5
2
PGNDC
10
Power Ground for the Core Converter. Connect all grounds together close to the IC.
6
3
PGND
Power Ground. Ground for NDRV and core output synchronous rectifier. Connect all grounds
together close to the IC.
7
4
NDRV
N-Channel Drive Output. Drives the main output synchronous-rectifier MOSFET. NDRV swings
between CVL and PGND.
Low-Side Bypass. CVL is the output of an internal LDO regulator. This is the internal power supply
for the device control circuitry as well as the N-channel driver. Bypass CVL with a 1.0µF or greater
capacitor to GND. When CS- is above the CVL switchover threshold (2.47V), CVL is powered from
the main output.
8
5
CVL
9
6
IN
10
7
PDRV
P-Channel Drive Output. Drives the main output high-side MOSFET switch. PDRV swings between IN and
CVH. The voltage at CVH is regulated at VIN - 4.2V unless the input voltage is less than 5.5V.
11
8
CVH
High-Side Drive Bypass. This is the low-side of the P-channel driver output. Bypass with a 1.0µF
capacitor or greater to IN. When the input voltage is less than 5.5V, CVH is switched to PGND.
12
9
LXB
Backup Converter Switching Node. Connect an inductor from LXB to the backup battery and a
Schottky diode to BIN to complete the backup converter. In backup mode, this step-up converter
powers the main output from the backup battery through BIN.
Power Supply Input
______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
Pin Description (continued)
NAME
FUNCTION
QSOP
QFN
—
10
LXB2
13
11
BIN
14
12
BKOFF
Backup Disable Input. Driving BKOFF below +0.5V disables the backup mode. In backup mode, the
device enters shutdown when this pin is pulled low. BKOFF can be driven from a digital signal or can be
used as a low battery detector to disable the backup converter when the backup battery is low.
15
13
ACI
AC Adapter Low-Voltage Detect Input. Connect to adapter DC input. When the voltage at ACI falls
below the voltage at INS plus +0.22V, ACO asserts.
16
14
DBI
Dead Battery Input. Connect DBI to the main battery through a resistive voltage-divider. When DBI drops
below +1.20V, no AC adapter is connected (ACO is low, but main output still available), BKUP asserts.
17
15
LBI
Low-Battery Input. Connect LBI to the main battery through a resistive voltage-divider. When the
voltage at LBI drops below +1.20V, LBO asserts.
18
16
REF
Reference Voltage Output. Bypass REF to GND with a 0.22µF or greater capacitor.
—
17, 25,
29
N.C.
19
18
FBM
Main Output Feedback. Connect FBM to a resistive voltage-divider to set main output voltage
between +1.25V to +5.5V.
20
19
CS+
Main Regulator High-Side Current-Sense Input. Connect the sense resistor between CS+ and CS-.
This voltage is used to set the current limit and to turn off the synchronous rectifier when the
inductor current approaches zero.
21
20
CS-
Main Regulator Low-Side Current-Sense Input. Connect CS- to the main output.
22
21
FBC
Core Output Feedback. Connect FBC to a resistive voltage-divider to set core output between +1.0V
to +5.0V.
23
22
GND
Analog Ground
24
23
INC
Core Supply Input
25
24
ACO
Low AC Output. Open drain ACO asserts when ACI falls below the main output voltage plus 0.22V.
26
26
LBO
Open-Drain Low-Battery Output. LBO asserts when LBI falls below +1.20V.
Backup Converter Switching Node. Connect LXB2 to LXB as close to the IC as possible.
Backup Battery Input. Connect BIN to the output of the backup boost regulator. Bypass BIN with a 10µF or
greater capacitor to GND. When the MAX1774 is in backup mode, BIN powers the main output.
No Connection. Not Internally Connected.
27
27
INS
Power-Supply Input Voltage Sense Input. Connect INS to the power-supply input voltage.
28
28
LXC
Core Converter Switching Node
Detailed Description
The MAX1774 dual step-down DC-DC converter is
designed to power PDA, palmtop, and subnotebook
computers. Normally, these devices require two separate power supplies–one for the processor and another
higher voltage supply for the peripheral circuitry. The
MAX1774 provides an adjustable +1.25V to +5.5V main
output designed to power the peripheral circuitry of
PDAs and similar devices. The main output delivers up
to 2A output current. The lower voltage core converter
has an adjustable +1.0V to +5.0V output, providing up
to 1.5A output current. Both regulators utilize a proprietary regulation scheme allowing PWM operation at
medium to heavy loads, and automatically switch to
pulse skipping at light loads for improved efficiency.
Under low-battery conditions, the MAX1774 enters
backup mode, making use of a low-voltage backup
battery and a step-up regulator to power the output.
Figure 1 is the MAX1774 typical application circuit.
Operating Modes for the
Step-Down Converters
When delivering low output currents, the MAX1774 operates in discontinuous conduction mode. Current through
the inductor starts at zero, rises as high as the minimum
current limit (IMIN), then ramps down to zero during
_______________________________________________________________________________________
11
MAX1774
PIN
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
NOTE: FOR INPUT VOLTAGES
TO 28V SEE FIGURE 4
AND FIGURE 5
2.7V
TO
5.5V
D1
2.7V V
IN_AC
TO
5.5V
MAIN
BATTERY
NSD03A10
C5
1µF
NDS356AP
P1
INS
IN
CVH
R4
R1
C6
10µF
MDRV
P2
DBI
PDRV
ACI
R2
FDS8928A
L1
5µH
RCS
MAIN
N1
LBI
MAX1774
1MΩ
R3
CMAIN
47µF
NDRV
1.25V
TO
5.5V
PGND
ON
SHDNM
CS+
ON
SHDNC
CS-
OFF
OFF
BIN
C1
10µF
R10
D2
EP05Q
03L
LXB
FBM
LXB2(QFN ONLY)
L3
22µH
BACKUP
BATTERY
0.9V
TO
5.5V
R11
BKOFF
C2
10µF
R5
1MΩ
ACO
CVL
C3
1µF
R6
1MΩ
C4
0.22µF
REF
LBO
R7
1MΩ
GND
BKUP
INC
PGNDC
C7
1µF
L2
5.4µH
CORE
LXC
R8
1.0V
TO
CCORE 5.5V
22µF
FBC
R9
Figure 1. Typical Application Circuit For Low-Input Voltage Applications
each cycle (see Typical Operating Characteristics). The
switch waveform may exhibit ringing, which occurs at
the resonant frequency of the inductor and stray capacitance, due to the residual energy trapped in the core
when the rectifier MOSFET turns off. This ringing is normal and does not degrade circuit performance.
When delivering medium-to-high output currents, the
MAX1774 operates in PWM continuous-conduction
mode. In this mode, current always flows through the
inductor and never ramps to zero. The control circuit
12
adjusts the switch duty cycle to maintain regulation
without exceeding the peak switching current set by
the current-sense resistor.
100% Duty Cycle and Dropout
The MAX1774 operates with a duty cycle up to 100%,
extending the input voltage range by turning the MOSFET on continuously when the supply voltage approaches the output voltage. This services the load
when conventional switching regulators with less than
______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
MAX1774
TOFFMIN
VMIN
CS+
CS-
TONMIN
PON
VVALLEY
S
FB
PON
VIN
Q
PSW
REF
R
VCLM
S
VO
Q
NON
NSW
R
VZERO
NONOVERLAP
PROTECTION
Figure 2. Simplified Control System Block Diagram
100% duty cycle fail. Dropout voltage is defined as the
difference between the input and output voltages when
the input is low enough for the output to drop out of
regulation. Dropout depends on the MOSFET drain-tosource on-resistance, current-sense resistor, and
inductor series resistance, and is proportional to the
load current:
VDROPOUT = IOUT [RDS(ON) + RSENSE + RL]
Regulation Control Scheme
The MAX1774 has a unique operating scheme that
allows PWM operation at medium and high current,
automatically switching to pulse-skipping mode at
lower currents to improve light-load efficiency. Figure 2
shows a simplified block diagram.
Under medium and heavy load operation, the inductor
current is continuous and the part operates in PWM
mode. In this mode, depending on the duty cycle,
either the minimum on-time or the minimum off-time
sets the switching frequency. The duty cycle is approximately the output voltage divided by the input voltage.
If the duty cycle is less than 50%, the minimum on-time
controls the frequency, and the frequency is approximately f ≈ 2.5MHz ✕ D, where D is the duty cycle. If the
duty cycle is greater than 50%, the minimum off-time
sets the frequency, and the frequency is approximately
f ≈ 2.5MHz ✕ (1 - D).
In both cases, the error comparator regulates the voltage. For low duty cycles (<50%), the P-channel MOSFET is turned on for the minimum on-time, causing
fixed-on-time operation. During the MOSFET on-time,
the output voltage rises. Once the MOSFET is turned
off, the voltage drops to the regulation threshold, when
another cycle is initiated. For high duty cycles (>50%),
the MOSFET remains off for the minimum off-time,
causing fixed-off-time operation. In this case, the MOSFET remains on until the output voltage rises to the regulation threshold. Then the MOSFET turns off for the
minimum off-time, initiating another cycle.
By switching between fixed-on-time and fixed-off-time
operation, the MAX1774 can operate at high input-output ratios and still operate up to 100% duty cycle for
low dropout. When operating from fixed-on-time operation, the minimum output voltage is regulated, but in
fixed-off-time operation, the maximum output voltage is
regulated. Thus, as the input voltage drops below
approximately twice the output voltage, a decrease in
line regulation can be expected. The drop in voltage is
approximately VDROP ≈ VRIPPLE. At light output loads,
the inductor current is discontinuous, causing the
MAX1774 to operate at lower frequencies, reducing the
MOSFET gate drive and switching losses. In discontinuous mode, under most circumstances, the on-time will
be a fixed minimum on-time of 400ns.
______________________________________________________________________________________
13
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
The MAX1774 features four separate current-limit
threshold detectors and a watchdog timer for each of its
step-down converters. In addition to the more common
peak-current detector and zero-crossing detector, each
converter also provides a valley-current detector, and a
minimum-current detector. The valley-current detector is
used to force the inductor current to drop to a lower
level after hitting peak current before allowing the Pchannel MOSFET to turn on. This is a safeguard against
inductor current significantly overshooting above the
peak current when the inductor discharges too slowly
when VOUT/L is small. The minimum-current detector
ensures that a minimum current is built up in the inductor before turning off the P-channel MOSFET. This helps
the inductor to charge the output near dropout when
the dl/dt is small (because (VIN - VOUT) / L is small) to
avoid multiple pulses and low efficiency. This feature,
however, is disabled during dropout and light-load conditions where the inductor current may take too long to
reach the minimum current value. A watchdog timer
overrides the minimum current after the P-channel MOSFET has been on for longer than about 10µs.
Main Step-Down Converter
The main step-down converter features adjustable
+1.25V to +5.5V output delivering up to 2A from a
+2.7V to +28V input (see Setting the Output Voltages ).
The use of external MOSFETs and current-sense resistor maximizes design flexibility. The MAX1774 offers a
synchronous-rectifier MOSFET driver that improves efficiency by eliminating losses through a diode. The two
MOSFET drive outputs, PDRV and NDRV, control these
external MOSFETs. The output swing of these outputs
is limited to reduce power consumption by limiting the
amount of injected gate charge (see Internal Linear
Regulators section for details). Current-limit detection
for all main converter current limits is sensed through a
small-sense resistor at the converters’ output (see
Setting the Current Limit section ). Driving the SHDNM
pin low puts the main converter in a low-power shutdown mode. The core regulator, low-voltage detectors,
and backup converter are still functional when the main
converter is in shutdown. When the MAX1774 enters
backup mode, the main converter and its current sensor are shut off.
Core Step-Down Converter
The core step-down converter produces a +1.0V to
+5.0V output from a +2.6V to +5.5V input. The low-voltage input allows the use of internal power MOSFETs,
taking advantage of their low RDS(ON), improving efficiency and reducing board space. Like the main converter, the core regulator makes use of a synchronousrectifying N-channel MOSFET, improving efficiency and
14
eliminating the need for an external Schottky diode.
Current sensing is internal to the device, eliminating the
need for an external sense resistor. The maximum and
minimum current limits are sensed through the P-channel MOSFET, while the valley current and zero-crossing
current are sensed through the N-channel MOSFET.
The core output voltage is measured at FBC through a
resistive voltage-divider. This divider can be adjusted
to set the output voltage level (see Setting the Output
Voltages). The core input can be supplied from the
main regulator or an external supply that does not
exceed +5.5V (see High-Voltage Configuration and
Low-Voltage Configuration sections). The core converter can be shut down independent of the main converter
by driving SHDNC low. If the main converter output is
supplying power to the core and is shut down, SHDNM
controls both outputs. In this configuration, the core
converter continues to operate when the MAX1774 is in
backup mode.
Voltage Monitors and Battery Switchover
The MAX1774 offers voltage monitors ACI, LBI, DBI,
and BKOFF that drive corresponding outputs to indicate low-voltage conditions. The AC adapter low-voltage detect input, ACI, is typically connected to the
output of an AC-to-DC converter. When the voltage at
ACI drops below the INS sense input plus 0.22V, the
low AC output, ACO, is asserted. Figure 3 shows a simplified block diagram.
The low and dead battery monitors (LBI and DBI) monitor the voltage at MAIN_BATT through a resistive voltage-divider. When the voltage at LBI falls below
+1.20V, the low-battery output flag, LBO, is asserted.
When both VIN_AC and MAIN_BATT are present, the
MAX1774 chooses one of the two supplies determined
by ACI. To facilitate this, the MAX1774 provides an
open-drain MOSFET driver output (MDRV). This drives
an external P-channel MOSFET used to switch the
MAX1774 from the AC input to the battery. MDRV goes
low when ACO is low, the main battery is not dead, and
the MAX1774 is not in backup mode.
The MAX1774 enters backup mode when the voltage at
DBI is below +1.20V and VIN_AC is not present to the
board. Under these conditions, the BKUP output is
asserted (low), and the device utilizes its boost converter and a low-voltage backup battery to supply the main
output. The BKUP pin can be driven low externally,
forcing the MAX1774 to enter backup mode. If the voltage at BKOFF is less than 0.5V, the backup converter
is disabled. BKOFF can be driven from a digital signal,
or can be used as a low-battery detector to disable the
backup converter when the backup battery is low.
______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
MAX1774
LBO
LBO
LBI
1.2V
MDRV
MDRV
DBI
DBO
1.2V
BKUP
BKUP
ACI
ACO
INS
NOAC
0.22V
BKUP
MODE
BKOFF
0.5V
CS- (MAIN OUT)
BIN
CS+
MAIN
RDY
CVL
CS- CS+
IN
RDY
REF
REF
CVL
PDRV
MAIN
BUCK
EN
CVH
CVH
FB
SOFT-START
NDRV
ON
PGND
SHDNM
SHDNC
LXB2
(QFN ONLY)
ON
INC
LXB
BACKUP
BOOST
EN
CORE
BUCK
LXC
FB
PGND
FB
PGNDC
GND
MAX1774
FBM
FBC
Figure 3. Simplified Block Diagram
______________________________________________________________________________________
15
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
Place 1MΩ pullup resistors from the main output to
ACO, LBO, and BKUP. Use a 1MΩ pullup resistor from
MDRV to IN.
When not in backup mode, the backup regulator is isolated from the main output by an internal switch. When
the MAX1774 is in backup mode, the main converter is
disabled, and the output of the backup regulator is
connected to the main output. The core converter is still
operable while in backup mode. The backup step-up
converter cannot drive the typical main load current.
The load at main must be reduced before entering
backup mode.
If BKUP is de-asserted (goes high), the MAX1774 exits
backup mode and resumes operation from the main
battery or the AC adapter input. If BKOFF goes low, or
the backup battery discharges where it cannot sustain
the main output load, the backup converter shuts off.
To restart the main converter, apply power to VIN_AC or
MAIN_BATT.
The backup converter uses an external Schottky diode
and internal power NMOS switch. Since this converter
shares the same output as the main buck converter, it
shares the same feedback network. This automatically
sets the backup converter output voltage to that of the
main converter. The backup converter generates an
output between +1.25V and +5.5V from a +0.9V to
+5.5V input, and provides a load current up to 20mA.
When the MAX1774 is in backup mode, the main current- sense circuit is turned off to conserve power.
When the output is out of regulation, the maximum
inductor current limit and zero-current detectors regulate switching. The N-channel MOSFET is turned on
until the maximum inductor current limit is reached, and
shuts off until the inductor current reaches zero. When
the output is within regulation, switching is controlled
by the maximum pulse width, LXB, switch current limit,
zero crossing, and the feedback voltage.
Internal Linear Regulators
There are two internal linear regulators in the MAX1774.
A high-voltage linear regulator accepts inputs up to
+28V, reducing it to +2.8V at CVL to provide power to
the MAX1774. If the voltage at CS- is greater than
+2.47V, CVL is switched to CS-, allowing it to be driven
from the main converter, improving efficiency. CVL supplies the internal bias to the IC and power for the NDRV
gate driver.
The CVH regulator output provides the low-side voltage
for the main regulator’s PDRV output. The voltage at
CVH is regulated at 4.2V below VIN to limit the voltage
swing on PDRV, reducing gate charge and improving
efficiency (Figure 3).
16
Reference
The MAX1774 has a trimmed internal +1.25V reference
at REF. REF can source no more than 50µA. Bypass
REF to GND with a 0.22µF capacitor.
Design Procedure
Low-Voltage Configuration
To improve efficiency and conserve board space, the
core regulator operates from low input voltages, taking
advantage of internal low-voltage, low-on-resistance
MOSFETs. When the input voltage remains below 5.5V,
run the core converter directly from the input by connecting INC to IN (Figure 1). This configuration takes
advantage of the core’s low-voltage design and
improves efficiency.
High-Voltage Configuration
For input voltages greater than 5.5V, cascade the main
and core converters by connecting INC to the main output voltage (Figure 4). In this configuration, the core
converter is powered from the main output. Ensure that
the main output can simultaneously supply its load and
the core input current.
Backup Converter Configuration
The MAX1774 provides a backup step-up converter to
power the device and provide the main output voltage
when other power fails. The backup converter operates
from a +0.9V to +5.5V battery. For most rechargeable
batteries, such as NiCd or NiMH, the simple circuit of
Figure 5 can be used to recharge the backup battery.
In this circuit, the backup battery is charged through
R1 and D10. Consult the battery manufacturer for
charging requirements. To prevent the backup battery
from overdischarging, connect a resistive voltagedivider from the backup battery to BKOFF. Resistor values can be calculated through the following equation:
R12 = R13 ✕ [(VBU / V BKOFF) - 1]
where V BKOFF = 0.5V, and VBU is the minimum acceptable backup battery voltage. Choose R13 to be less
than 150kΩ.
Setting the Output Voltages
The main output voltage is set from +1.25V and +5.5V
with two external resistors connected as a voltagedivider to FBM (Figure 1). Resistor values can be calculated by the following equation:
R10 = R11 ✕ [(VOUTM / VFBM) - 1]
where VFBM = +1.25V. Choose R11 to be 40kΩ or less.
The core regulator output is adjustable from +1.0V to
+5.0V through two external resistors connected as a
______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
D1
VIN_AC
MAIN
BATTERY
2.7V
TO
20V
NSD03A10
C5
1µF
NDS356AP
P1
1MΩ
MAX1774
2.7V TO 28V
IN
ACI
INS
CVH
R4
R1
C6
10µF
MDRV
P2
DBI
FDS8928A
L1
5µH
PDRV
R2
RCS
MAIN
N1
LBI
CMAIN
47µF
NDRV
2.6V
TO
5.5V
MAX1774
R3
PGND
ON
SHDNM
CS+
ON
SHDNC
CS-
OFF
OFF
BIN
C1
10µF
LXB
LXB2 (QFN ONLY)
FBM
L3
22µH
BACKUP
BATTERY
0.9V
TO
5.5V
R10
D2
EP05
Q03L
R11
BKOFF
C2
10µF
CVL
R5
1MΩ
ACO
C3
1µF
R6
1MΩ
C4
0.22µF
REF
LBO
R7
1MΩ
GND
PGNDC
BKUP
INC
C7
1µF
L2
5.4µH
CORE
LXC
R8
1.0V
CCORE TO
22µF 5.5V
FBC
R9
Figure 4. Typical Application Circuit (Cascaded)
voltage-divider to FBC (Figure 1). Resistor values can
be calculated with the following equation:
R8 = R9 ✕ [(VOUTC / VFBC) - 1]
where VFBC = +1.0V. Choose R9 to be 30kΩ or less.
Setting the Current Limit
The main regulator current limit is set externally through
a small current-sense resistor, R CS (Figure 1). The
value of RCS can be calculated with the following equation:
RCS = VCLM / (1.3 ✕ IOUT)
where VCLM = 80mV is the current-sense threshold,
and IOUT is the current delivered to the output. The
core and backup converter current limits are set internally and cannot be modified.
Careful layout of the current-sense signal traces is
imperative. Place RCS as close to the MAX1774 as possible. The two traces should have matching length and
width, be as far as possible from noisy switching sig-
______________________________________________________________________________________
17
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
2.7V
TO
28V
D1
VIN_AC
MAIN
BATTERY
2.7V
TO
20V
NSD03A10
C5
NDS356AP
IN
INS
CVH
R4
P1
C6
10µF
R1
MDRV
P2
DBI
FDS8928A
L1
10µH
PDRV
R2
RCS
MAIN
N1
LBI
ACI
R3
CMAIN
47µF
NDRV
2.6V
TO
5.5V
MAX1774
1MΩ
PGND
R12
ON
SHDNM
CS+
ON
SHDNC
CS-
OFF
OFF
BIN
D2
EP05
Q03L
C1
10µF
R10
LXB
FBM
LXB2 (QFN ONLY)
L3
22µH
R11
R13
R5
1MΩ
BKOFF
BACKUP
BATTERY
ACO
C2
10µF
0.9V
TO
5.5V
R6
1MΩ
CVL
LBO
C3
R7
1MΩ
REF
C4
0.22µF
GND
BKUP
INC
C7
1µF
PGNDC
L2
CORE
LXC
R8
CCORE
22µF
1.0V
TO
5.5V
FBC
R9
Figure 5. Typical Application Circuit (with Recharge)
nals, and be close together to improve noise rejection.
These traces should be used for current-sense signal
routing only and should not carry any load current.
Refer to the MAX1774 evaluation kit for layout examples.
Setting the Voltage Monitor Levels
The low battery and dead battery detector trip points
can be set by adjusting the resistor values of the
18
divider string (R1, R2, and R3) in Figure 1 according to
the following equations:
R1 = (R2 + R3) ✕ [(VBD / VTH) - 1]
R2 = R3 ✕ [(VBL / VBD) - 1]
where VBL is the low battery voltage, VBD is the dead
battery voltage, and VTH = +1.20V. Choose R3 to be
less than 250kΩ.
______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
The inductor’s saturation current must be greater than
the peak switching current to prevent core saturation.
Saturation occurs when the inductor’s magnetic flux
density reaches the maximum level the core can support and inductance starts to fall. The inductor heating
current rating must be greater than the maximum load
current to prevent overheating. For optimum efficiency,
the inductor series resistance should be less than the
current-sense resistance.
Capacitor Selection
Choose the output filter capacitors to service input and
output ripple current with acceptable voltage ripple.
ESR in the output capacitor is a major contributor to
output ripple. For the main converter, low-ESR capacitors such as polymer or ceramic capacitors are recommended. For the core converter, choosing a low-ESR
tantalum capacitor with enough ESR to generate about
1% ripple voltage across the output is helpful in ensuring stability.
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
VRIPPLE ≈ VRIPPLE,ESR + VRIPPLE,C
For tantalum capacitors, the ripple is determined mostly
by the ESR. Voltage ripple due to ESR is:
VRIPPLE,ESR ≈ (RESR) ✕ IRIPPLE
For ceramic capacitors, the ripple is mostly due to the
capacitance. The ripple due to the capacitance is
approximately:
VRIPPLE,C ≈ L IRIPPLE2COUT VOUT
where VOUT is the average output voltage.
These equations are suitable for initial capacitor selection. Final values should be set by testing a prototype
or evaluation kit. When using tantalum capacitors, use
good soldering practices to prevent excessive heat
from damaging the devices and increasing their ESR.
Also, ensure that the tantalum capacitors’ surge-current
ratings exceed the startup inrush and peak switching
currents.
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple at IN, caused by the circuit’s switching. Use a
low-ESR capacitor. Two smaller value low-ESR capacitors can be connected in parallel if necessary. Choose
input capacitors with working voltage ratings higher
than the maximum input voltage.
MOSFET Selection
The MAX1774 drives an external enhancement-mode Pchannel MOSFET and a synchronous-rectifier N-channel
MOSFET. When selecting the MOSFETs, important parameters to consider are on-resistance (RDS(ON)), maximum drain-to-source voltage (V DS(MAX) ), maximum
gate-to-source voltage (V GS(MAX) ), and minimum
threshold voltage (VTH(MIN)).
Chip Information
TRANSISTOR COUNT: 4545
PROCESS: BiCMOS
Pin Configurations (continued)
TOP VIEW
SHDNM 1
28 LXC
SHDNC 2
27 INS
BKUP 3
26 LBO
MDRV 4
25 ACO
PGNDC 5
PGND 6
24 INC
MAX1774
23 GND
NDRV 7
22 FBC
CVL 8
21 CS-
IN 9
20 CS+
PDRV 10
19 FBM
CVH 11
18 REF
LXB 12
17 LBI
BIN 13
16 DBI
BKOFF 14
15 ACI
28 QSOP
______________________________________________________________________________________
19
MAX1774
Inductor Selection
The essential parameters for inductor selection are
inductance and current rating. The MAX1774 operates
with a wide range of inductance values.
Calculate the inductance value for either CORE or
MAIN, LMIN :
L(MIN) = (VIN - VOUT) ✕ (tON(MIN) / lRIPPLE)
where tONMIN is typically 400ns, and lRIPPLE is the continuous conduction peak-to-peak lRIPPLE current.
In continuous conduction, lRIPPLE should be chosen to
be 30% of the maximum load current. With high inductor values, the MAX1774 begins continuous-conduction
operation at a lower fraction of full load (see Detailed
Description).
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
QFN 28, 32,44, 48L.EPS
MAX1774
Package Information
20
______________________________________________________________________________________
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
QSOP.EPS
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.
21 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX1774
Package Information (continued)
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