Page 1
General Description
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.
________________________Applications
Hand-Held Computers
PDAs
Internet Access Tablets
POS Terminals
Subnotebooks
Features
♦ Dual, High-Efficiency, Synchronous-Rectified
Step-Down Converters
♦ 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
♦ Input Voltage Range from +2.7V to +28V
♦ Four Low-Voltage Detectors
♦ 170µA Quiescent Current
♦ 8µA Shutdown Current
♦ Digital Soft-Start
♦ Independent Shutdown Inputs
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
________________________________________________________________ Maxim Integrated Products 1
32
31
30
29
28
27
26
BKUP
SHDNC
SHDNM
N.C.
LXC
INS
LBO
25 N.C.
9
10
11
12
13
14
15
LXB
LXB2
BIN
BKOFF
ACI
DBI
LBI
16REF
17
18
19
20
21
22
23
N.C.
GND
GND
GND
GND
FBM
CS+
CS-
FBC
GND
INC
8
7
6
5
4
3
2
CVH
PDRV
IN
CVL
NDRV
PGND
PGNDC
MAX1774
32 7mm x 7mm QFN
1MDRV
24 ACO
TOP VIEW
Pin Configurations
19-1810; Rev 1; 1/02
EVALUATION KIT
AVAILABLE
Ordering Information
PART
PIN-PACKAGE
MAX1774EEI
28 QSOP
MAX1774EMJ
32 7mm x 7mm QFN
Functional Diagram
Pin Configurations continued at end of data sheet.
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.
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
AC
ADAPTER
MAIN
BATTERY
BACKUP
BATTERY
MAX1774
AC OK
LOW MAIN BATTERY
DEAD MAIN BATTERY
MAIN (+3.3V)
CORE (+1.8V)
Page 2
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(Figure 1, VIN= V
INS
+12V, V
INC
= V
CS-
= V
CS+
= +3.3V, V
CORE
= +1.8V, TA= 0°C to +85°C, unless otherwise noted. Typical values are at
T
A
= +25°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.
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 (V
BIN+ 0.7V)
PDRV to GND..................................(V
CVH
- 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
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Input Voltage V
IN
2.7 28 V
Input Quiescent Supply Current I
IN
V
FBM
= +1.5V, V
FBC
= +1.5V,
V
SHDNM
= V
SHDNC
= +3.3V
18 40 µA
CS- Quiescent Supply Current I
CS-
V
FBM
= +1.5V, V
FBC
= +1.5V,
V
SHDNM
= V
SHDNC
= +3.3V
µA
Core Regulator Quiescent
Supply Current
I
INC
V
FBM
= +1.5V, V
FBC
= +1.5V,
V
SHDNM
= V
SHDNC
= +3.3V
60
µA
Backup Mode BIN Quiescent
Supply Current
I
BIN
V
BIN
= +3.3V, CS- open
V
FBM
= +1.5V, V
SHDNM
= +3.3V,
V
BKOFF
= +1.5V, SHDNC = GND
60
µA
IN Shutdown Supply Current SHDNM = SHDNC = GND 8 40 µA
MAIN REGULATOR
Main Output Voltage Adjust
Range
5.5 V
FBM Regulation Threshold V
FBM
V
(CS+ - CS-)
= 0 to +60mV,
V
IN
= +3.5V to +28V
V
FBM Input Current I
FBM
V
FBM
= +1.3V
0.1 µA
Current-Limit Threshold V
CS+
- V
CS-
60 80
mV
Minimum Current-Limit
Threshold
V
CS+
- V
CS-
51525mV
Valley Current Threshold V
CS+
- V
CS-
40 50 60 mV
Zero Current Threshold V
CS+
- V
CS-
0515mV
PDRV, NDRV Gate Drive
Resistance
V
CS-
= +3.3V, I
PDRV
, I
NDRV
= 50mA 2 5.5 Ω
CS- to CVL Switch Resistance I
CVL
= 50mA 4.5 9.5 Ω
PDRV, NDRV Dead Time 50 ns
110 220
1.25
1.21 1.25 1.29
-0.1
105
105
100
Page 3
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
_______________________________________________________________________________________ 3
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Maximum Duty Cycle
%
Minimum On-Time
ns
Minimum Off-Time
ns
CORE REGULATOR
Input Voltage Range V
INC
2.6 5.5 V
V
INC
rising
INC Undervoltage Lockout
V
INC
falling
V
C or e Outp ut V ol tag e Ad j ust Rang e
1.0 5.0 V
Maximum Core Load Current V
CORE
= 1.8V (Note 1) 1 1.5 A
FBC Regulation Threshold V
FBC
V
INC
= +2.5 to +5.5V, I
O U T C
= 0 to 200mA
1.0
V
FBC Input Current I
FBC
V
FBC
= +1.3V
0.1 µA
Dropout Voltage I
OUTC
= 400mA 0.1
V
LXC Leakage Current I
LXC
V
INC
= +5.5V, V
L X C
= 0 to +5.5V -10 10 µA
LXC P-Channel, N-Channel OnResistance
0.5 Ω
LXC P-Channel Current Limit I
CLC
mA
LX C P - C hannel M i ni m um C ur r ent
mA
LXC N-Channel Valley Current
mA
LXC N-Channel Zero-Crossing
Current
40
mA
LXC Dead Time 50 ns
Max Duty Cycle
%
Minimum On-Time
ns
Minimum Off-Time
ns
BACKUP REGULATOR
Backup Battery Input Voltage
0.9 5.5 V
LXB N-Channel On-Resistance V
CS-
= +3.3V, I
LXB
= 50mA 1.9 3.5 Ω
LXB Current Limit
mA
LXB Leakage Current V
LXB
= +5.5V, V
FBM
= +1.3V 1 µA
BIN Leakage Current I
BIN
V
BIN
= +5.5V, CS- = BKOFF =
SHDNC = SHDNM = GND
1 µA
BIN, CS- Switch Resistance
V
CS-
= +3.3V, BKOFF = GND,
SHDNM = CVL
7.5 15 Ω
BIN Switch Zero-Crossing
Threshold
V
BIN
= + 2.5V , BKOFF = SHDNC =
SHDNM = C V L
17 35 mV
LXB Maximum On-Time 2.8 5.6 9.2 µs
Zero Crossing Detector Timeout
40 µs
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN= V
INS
= +12V, V
INC
= V
CS-
= V
CS+
= +3.3V, V
CORE
= +1.8V, TA= 0°C to +85°C, unless otherwise noted. Typical values are
at T
A
= +25°C.)
V
BBATT
100
200 400 650
200 400 650
2.40 2.47 2.55
2.30 2.37 2.45
0.97
-0.1
1200 1800 3000
100 250 400
900 1400 2400
100
170 400 690
170 400 690
200 350 600
0.25
110 170
1.03
0.25
Page 4
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
4 _______________________________________________________________________________________
PARAMETER
CONDITIONS
UNITS
REFERENCE
Reference Voltage V
REF
V
Reference Load Regulation I
REF
= 0 to 50µA 10 mV
Reference Line Regulation V
CS-
= +2.5V to +5.5V, I
REF
= 50µA 5 mV
Reference Sink Current 10 µA
CVL, CVH REGULATORS
I
CVL
= 50mA, V
CS-
= 0 2.6 2.8 3.1
CVL Output Voltage V
CVL
I
CVL
= 50mA, V
CS-
= +3.3V 3.2
V
CVL Switchover Threshold CS- rising, hysteresis = 100mV typical
V
VIN = +4V, I
CVH
= 25mA
V
IN
-
3.4
V
IN
-
2.8
CVH Output Voltage V
CVH
VIN = +12V, I
CVH
= 50mA
V
IN
-
4.2
V
IN
-
3.7
V
CVH Switchover Threshold V
IN
VIN rising, hysteresis = 350mV typ 5.5 V
V
CVL
rising
CVL Undervoltage Lockout
V
CVL
falling
V
LOW-VOLTAGE COMPARATORS
V
BKOFF
rising
Backup Regulator Shutdown
Threshold
V
BKOFF
falling
V
BKOFF Input Bias Current V
BKOFF
= +5.5V 1 µA
LBI Threshold V
LBI
V
LBI
falling, hysteresis = 50mV typical
V
DBI Threshold V
DBI
V
DBI
falling, hysteresis = 50mV typical
V
BKUP Low-Input Threshold 0.4 V
LBI, DBI Input Leakage Current V
LBI
= V
DBI
= +1.3V
nA
LBO, BKUP, ACO, MDRV
Output Low
I
SINK
= 1mA 0.4 V
LBO, BKUP, ACO, MDRV
Output Leakage Current
V
LBI
= +1.3V, V
ACI
= +12V, V
ACO
=
V
LBO
= V
BKUP
= +5.5V, V
MDRV
= +28V
1.0 µA
ACI Threshold V
ACI
– V
INS,
ACI falling
V
ACI Input Leakage Current V
ACI
= +1.3V
nA
INS Input Leakage Current V
INS
= +3.3V 1.5 10 µA
LOGIC INPUTS
SHDNM, SHDNC Input Low
Voltage
0.4 V
SHDNM, SHDNC Input High
Voltage
2.0 V
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN= V
INS
= +12V, V
INC
= V
CS-
= V
CS+
= +3.3V, V
CORE
= +1.8V, TA= 0°C to +85°C, unless otherwise noted. Typical values are
at T
A
= +25°C.)
SYMBOL
MIN TYP MAX
1.23 1.25 1.27
2.40 2.47 2.55
2.40 2.47 2.55
V
BKOFF
2.30 2.37 2.45
0.51 0.55 0.59
0.46 0.50 0.54
1.17 1.20 1.23
1.17 1.20 1.23
0.22 0.35
100
100
Page 5
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
_______________________________________________________________________________________ 5
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SHDNM, SHDNC Input Low
Current
SHDNM = SHDNC = GND -1 1 µA
SHDNC Input High Current V
SHDNC
= +5.5V 5 µA
SHDNM Input High Current V
SHDNM
= +5V 2 25 µA
ELECTRICAL CHARACTERISTICS
(Figure 1, VIN= V
INS
= +12V, V
INC
= V
CS-
= V
CS+
= +3.3V, V
CORE
= +1.8V, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
Input Voltage V
IN
2.7 28 V
Input Quiescent Supply Current I
IN
V
FBM
= +1.5V, V
FBC
= +1.5V, V
SHDNM
=
V
SHDNC
= +3.3V
40 µA
CS- Quiescent Supply Current I
CS-
V
FBM
= +1.5V, V
FBC
= +1.5V, V
SHDNM
=
V
SHDNC
= +3.3V
µA
Core Regulator Quiescent
Supply Current
I
INC
V
FBM
= +1.5V, V
FBC
= +1.5V, V
SHDNM
=
V
SHDNC
= +3.3V
µA
Backup Mode BIN Quiescent
Supply Current
I
BIN
V
BIN
= +3.3V, CS- open
V
FBM
= +1.5V, V
SHDNM
= +3.3V,
V
BKOFF
= +1.5V, SHDNC = GND
µA
IN Shutdown Supply Current SHDNM = SHDNC = GND 40 µA
MAIN REGULATOR
Main Output Voltage Adjust
Range
5.5 V
FBM Regulation Threshold V
FBM
V
(CS+ - CS-)
= 0 to +60mV,
V
IN
= +3.5V to +28V
V
FBM Input Current I
FBM
V
FBM
= +1.3V
0.1 µA
Current-Limit Threshold V
CS+
- V
CS-
60
mV
Minimum Current-Limit
Threshold
V
CS+
- V
CS-
525mV
Valley Current Threshold V
CS+
- V
CS-
40 60 mV
Zero Current Threshold V
CS+
- V
CS-
015mV
PDRV, NDRV Gate Drive
Resistance
V
CS-
= +3.3V, I
PDRV
, I
NDRV
= 50mA 5.5 Ω
CS- to CVL Switch Resistance I
CVL
= 50mA 9.5 Ω
Maximum Duty Cycle
%
Minimum On-Time
ns
Minimum Off-Time
ns
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN= V
INS
= +12V, V
INC
= V
CS-
= V
CS+
= +3.3V, V
CORE
= +1.8V, TA= 0°C to +85°C, unless otherwise noted. Typical values are
at T
A
= +25°C.)
1.25
1.21 1.29
-0.1
100
200 650
200 650
220
105
110
100
Page 6
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
6 _______________________________________________________________________________________
PARAMETER
CONDITIONS
UNITS
CORE REGULATOR
Input Voltage Range V
INC
2.6 5.5 V
V
INC
rising
INC Undervoltage Lockout
V
INC
falling
V
Core Output Voltage Adjust
Range
1.0 5.0 V
Maximum Core Load Current V
CORE
= 1.8V (Note 1) 1 A
FBC Regulation Threshold V
FBC
V
INC
= +2.5 to +5.5V,
I
OUTC
= 0 to 200mA
V
FBC Input Current I
FBC
V
FBC
= +1.3V
0.1 µA
Dropout Voltage I
OUTC
= 400mA
V
LXC Leakage Current I
LXC
V
INC
= +5.5V, V
LXC
= 0 to +5.5V -10 10 µA
LXC P-Channel, N-Channel
On-Resistance
0.5 Ω
LXC P-Channel Current Limit
mA
LXC P-Channel Minimum
Current
mA
LXC N-Channel Valley Current
mA
LXC N-Channel Zero-Crossing
Current
40
mA
Max Duty Cycle
%
Minimum On-Time
ns
Minimum Off-Time
ns
BACKUP REGULATOR
Backup Battery Input Voltage
0.9 5.5 V
LXB N-Channel On Resistance V
CS-
= +3.3V, I
LXB
= 50mA 3.5 Ω
LXB Current Limit
mA
LXB Leakage Current V
LXB
= +5.5V, V
FBM
= +1.3V 1 µA
BIN Leakage Current I
BIN
V
BIN
= +5.5V, CS- = BKOFF =
SHDNC = SHDNM = GND
1µA
BIN, CS- Switch Resistance
V
CS-
= +3.3V, BKOFF = GND,
SHDNC = CVL
15 Ω
BIN Switch Zero-Crossing
Threshold
V
BIN
= +2.5V, BKOFF = SHDNC =
SHDNM = CVL
35 mV
LXB Maximum On-Time 2.8 9.2 µs
REFERENCE
Reference Voltage V
REF
V
Reference Load Regulation I
REF
= 0 to 50µA 10 mV
Reference Line Regulation V
CS-
= +2.5V to +5.5V, I
REF
= 50µA 5 mV
Reference Sink Current 10 µA
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN= V
INS
= +12V, V
INC
= V
CS+
= V
CS-
= +3.3V, V
CORE
= +1.8V, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
SYMBOL
MIN MAX
2.39 2.55
2.29 2.45
V
BBATT
0.97 1.03
-0.1
1200 3010
100 420
880 2450
100
160 700
170 690
200 600
1.220 1.275
0.25
170
Page 7
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
_______________________________________________________________________________________ 7
PARAMETER
CONDITIONS
UNITS
CVL, CVH REGULATORS
CVL Output Voltage V
CVL
I
CVL
= 50mA, V
CS-
= 0 2.6 3.1 V
CVL Switchover Threshold V
CS-
rising, hysteresis = 100mV typical
V
VIN = +4V, I
CVH
= 25mA
CVH Output Voltage V
CVH
VIN = +12V, I
CVH
= 50mA
V
V
CVL
rising
CVL Undervoltage Lockout
V
CVL
falling
V
LOW-VOLTAGE COMPARATORS
V
BKOFF
rising
Backup Regulator Shutdown
Threshold
V
BKOFF
falling
V
BKOFF Input Bias Current V
BKOFF
= +5.5V 1 µA
LBI Threshold V
LBI
V
LBI
falling, hysteresis = 50mV typical
V
DBI Threshold V
DBI
V
DBI
falling, hysteresis = 50mV typical
V
BKUP Low-Input Threshold 0.4 V
LBI, DBI Input Leakage Current V
LBI
, V
DBI
= +28V
nA
LBO, BKUP, ACO, MDRV
Output Low
I
SINK
= 1mA 0.4 V
LBO, BKUP, ACO, MDRV
Output Leakage Current
V
LBI
= +1.3V, V
ACI
= VIN = +12V, V
ACO
=
V
LBO
= V
BKUP
= +5.5V, V
MDRV
= +28V
1.0 µA
ACI Threshold V
ACI
- V
INS
, ACI falling 0.5 V
ACI Input Leakage Current V
ACI
= +1.3V
nA
MAIN Input Leakage Current V
INS
= +3.3V 10 µA
LOGIC INPUTS
SHDNM, SHDNC Input Low
Voltage
0.4 V
SHDNM, SHDNC Input High
Voltage
2.0 V
SHDNM, SHDNC Input Low
Current
SHDNM = SHDNC = GND -1 1 µA
SHDNC Input High Current V
SHDNC
= +5.5V 5 µA
SHDNM Input High Current V
SHDNM
= +28V 25 µA
ELECTRICAL CHARACTERISTICS (continued)
(Figure 1, VIN= V
INS
= +12V, V
INC
= V
CS+
= V
CS-
= +3.3V, V
CORE
= +1.8V, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
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.
SYMBOL
V
BKOFF
MIN MAX
2.40 2.55
VIN - 2.8
VIN - 3.65
2.40 2.57
2.30 2.47
0.51 0.59
0.46 0.54
1.17 1.23
1.17 1.23
100
100
Page 8
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
8 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 1, VIN= +5V, V
INC
= +3.3V, TA= +25°C, unless otherwise noted.)
100
0
1 10 100 1000 10,000
MAIN EFFICIENCY vs. LOAD
20
MAX1774-01
LOAD (mA)
EFFICIENCY (%)
40
60
80
70
50
30
10
90
VIN = +5V
VIN = +15V
V
IN
= +3.3V
VIN = +18V
VIN = +12V
V
MAIN
= 3.3V
90
0
1 100010010
CORE EFFICIENCY vs. LOAD
30
10
70
50
100
40
20
80
60
MAX1774-02
LOAD (mA)
EFFICIENCY (%)
VIN = +5V
VIN = +2.7V
VIN = +3.3V
V
CORE
= 1.8V
100
0
0.01 0.1 1 10 100
BACKUP EFFICIENCY vs. LOAD
20
MAX1774-03
LOAD (mA)
EFFICIENCY (%)
40
60
80
70
50
30
10
90
V
BBATT
= +0.8V
V
BBATT
= +1.0V
V
BBATT
= +2.5V
V
MAIN
= 3.3V
MAIN SWITCHING WAVEFORMS
(HEAVY LOAD 1A)
MAX1774-07
5µs/div
4V
LX
5V/div
0
20mV
0
-20mV
500mA
0
1000mA
1500mA
V
MAIN
(AC-COUPLED)
20mV/div
IL1
500mA/div
V
REF
ACCURACY vs. TEMPERATURE
MAX1774-04
-2.0
-1.5
-0.5
-1.0
1.0
1.5
0.5
0
2.0
V
REF
ACCURACY (%)
-40 0 20-20
40
60 80 100
TEMPERATURE (°C)
V
REF
ACCURACY (%)
-1.6
-1.8
-2.0
-1.2
-1.4
-0.8
-1.0
-0.6
-0.4
-0.2
0
0203010 40 50 60 70 80
REFERENCE LOAD REGULATION
MAX1774-05
I
REF
(µA)
MAIN SWITCHING WAVEFORMS
(LIGHT LOAD 100mA)
MAX1774-06
5µs/div
5V
LX
5V/div
0
40mV
20mV
0
V
MAIN
(AC-COUPLED)
20mV/div
I
LI
500mA/div
-20mV
500mA
0
Page 9
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
_______________________________________________________________________________________ 9
CORE SWITCHING WAVEFORMS
(HEAVY LOAD 500mA)
MAX1774-09
2µs/div
4V
2V
0
20mV
0
-20mV
500mA
0
V
CORE
(AC-COUPLED)
20mV/div
L2
500mA/div
LXC
2V/div
CORE LINE-TRANSIENT RESPONSE
MAX1774-11
V
INC
2V/div
0
2V
4V
V
CORE
(AC-COUPLED)
50mV/div
1µs/div
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= +5V, V
INC
= +3.3V, TA= +25°C, unless otherwise noted.)
CORE SWITCHING WAVEFORMS
(LIGHT LOAD 50mA)
MAX1774-08
1µs/div
3.3V
LX
2V/div
0
0
I
L2
500mA/div
500mA
V
CORE
(AC-COUPLED)
20mV/div
MAIN LOAD-TRANSIENT RESPONSE
MAX1774-12
1000mA
500mA
0
I
MAIN
500mA/div
-20mV
V
MAIN
(AC-COUPLED)
20mV/div
20mV
0
100µs/div
MAIN LOAD-TRANSIENT RESPONSE
50mA TO 500mA
MAX1774-13
500mA
0
V
MAIN
(AC-COUPLED)
10mV/div
I
MAIN
500mA/div
10mV
0
-10mV
100µs/div
MAIN LINE-TRANSIENT RESPONSE
MAX1774-10
12V
5V
0
50mV
-50mV
10V
V
IN
5V/div
V
MAIN
(AC-COUPLED)
50mV/div
100µs/div
Page 10
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
10 ______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= +5V, V
INC
= +3.3V, TA = +25°C, unless otherwise noted.)
TURN-ON RESPONSE
MAX1774-14
5V
0
V
OUT
1V/div
2V
1V
100µs/div
3V
0
400µA
200µA
0
INPUT
CURRENT
200mA/div
V
SHDN
5V/div
INPUT CURRENT
V
MAIN
V
CORE
BACKUP SWITCHOVER RESPONSE
MAX1774-15
V
BKUP
5V/div
I
BBATT
50mA/div
V
BIN
10mV/div
V
MAIN
10mV/div
5µs/div
Pin Description
PIN
QSOP
NAME FUNCTION
130
Shutdown for Main Regulator. Low voltage on SHDNM shuts off the main output. For normal
operation, connect SHDNM to IN.
231
Shutdown for Core Regulator. Low voltage on SHDNC shuts off the core output. For normal
operation, connect SHDNC to CVL.
332BKUP
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.
41MDRV
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.
52
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.
8 5 CVL
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.
9 6 IN Power Supply Input
10 7 PDRV
P - C hannel D r i ve Outp ut. D r i ves the m ai n outp ut hi g h- si d e M OS FE T sw i tch. P D RV sw i ng s b etw een IN and
C V H . The vol tag e at C V H i s r eg ul ated at V
IN
- 4.2V unl ess the i np ut vol tag e i s l ess 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.
QFN
SHDNM
SHDNC
PGNDC
Page 11
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
_______________________________________________________________________________________ 11
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 (I
MIN
), then ramps down to zero during
PIN
QSOP
NAME FUNCTION
— 10 LXB2 Backup Converter Switching Node. Connect LXB2 to LXB as close to the IC as possible.
13 11 BIN
Backup Batter y Inp ut. C onnect BIN to the outp ut of the b ackup b oos t r eg ul ator . Byp ass BIN w i th a 10µF or
g r eater cap aci tor to GN D . W hen the M AX 1774 i s i n b ackup m od e, BIN p ow er s the m ai n outp ut.
14 12
Backup D i sab l e Inp ut. D r i vi ng BKO FF b el ow + 0.5V d i sab l es the b ackup m od e. In b ackup m od e, the
d evi ce enter s shutd ow n w hen thi s p i n i s p ul l ed l ow . BKO FF can b e d r i ven fr om a d i g i tal si g nal or can b e
used as a l ow b atter y d etector to d i sab l e the b ackup conver ter w hen the b ackup b atter y i s l ow .
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
D ead Batter y Inp ut. C onnect D BI to the m ai n b atter y thr oug h a r esi sti ve voltage-divider. W hen D BI d r op s
b el ow + 1.20V , no AC ad ap ter i s connected ( ACO i s l ow , b ut m ai n outp ut sti l l avai l ab l e) , BKU P asser ts.
17 15 LBI
Low-Battery Input. Connect LBI to the main battery through a r esi sti ve 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. No Connection. Not Internally Connected.
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
C or e Outp ut Feed b ack. C onnec t FBC to a resistive voltage-divider to set cor e outp ut b etw een + 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.
27 27 INS Power-Supply Input Voltage Sense Input. Connect INS to the power-supply input voltage.
28 28 LXC Core Converter Switching Node
Pin Description (continued)
QFN
BKOFF
Page 12
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
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
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
12 ______________________________________________________________________________________
PGNDC
FBC
LXC
INC
FBM
CS-
CS+
PGND
PDRV
NDRV
CVH
MAIN
C5
1µF
P2
N1
L1
5µH
C
MAIN
47µF
C6
10µF
R
CS
CORE
C
CORE
22µF
R10
R11
R8
R9
L2
5.4µH
C7
1µF
R5
1MΩ
R6
1MΩ
R7
1MΩ
GND
REF
CVL
LXB
BIN
IN
DBI
LBI
BACKUP
BATTERY
MAIN
BATTERY
V
IN_AC
R1
C1
10µF
C2
10µF
C4
0.22µF
C3
1µF
D2
EP05Q
03L
L3
22µH
P1
R4
R2
R3
D1
BKOFF
BKUP
LBO
ACO
SHDNC
SHDNM
MDRV
MAX1774
ON
OFF
ON
OFF
1.0V
TO
5.5V
LXB2(QFN ONLY)
0.9V
TO
5.5V
1MΩ
ACI
INS
FDS8928A
1.25V
TO
5.5V
NDS356AP
NSD03A10
2.7V
TO
5.5V
2.7V
TO
5.5V
NOTE: FOR INPUT VOLTAGES
TO 28V SEE FIGURE 4
AND FIGURE 5
Figure 1. Typical Application Circuit For Low-Input Voltage Applications
Page 13
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
______________________________________________________________________________________ 13
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:
V
DROPOUT
= I
OUT
[R
DS(ON)
+ R
SENSE
+ 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 V
DROP
≈ V
RIPPLE.
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.
PSW
NON
PON
V
O
NSW
NONOVERLAP
PROTECTION
QS
R
S
R
Q
TONMIN
TOFFMIN
V
CLM
V
ZERO
FB
REF
CS+
CS-
V
VALLEY
V
MIN
V
IN
PON
Figure 2. Simplified Control System Block Diagram
Page 14
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 V
OUT
/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 (V
IN
- V
OUT
) / 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 R
DS(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
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 indi-
cate 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.
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
14 ______________________________________________________________________________________
Page 15
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
______________________________________________________________________________________ 15
Figure 3. Simplified Block Diagram
LBO
DBO
1.2V
LBI
DBI
ACI
BIN
CVL
REF
SHDNM
SHDNC
LXB
1.2V
INS
0.5V
0.22V
LBO
MDRV
BKUP
BKUP
MODE
ACO
CS- (MAIN OUT)
CS+
IN
PDRV
CVH
NDRV
PGND
INC
LXC
PGNDC
GND
FBC
RDY
CS-
MAIN
BUCK
ON
CS+
EN
FB
MDRV
BKUP
NOAC
ON
CORE
BUCK
PGND
BACKUP
BOOST
FB
FBM
EN
FB
CVH
CVLREF
SOFT-START
MAX1774
MAIN
RDY
BKOFF
LXB2
(QFN ONLY)
Page 16
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_ACor
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 VINto limit the voltage
swing on PDRV, reducing gate charge and improving
efficiency (Figure 3).
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 VBUis 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
✕
[(V
OUTM
/ V
FBM
) - 1]
where V
FBM
= +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
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
16 ______________________________________________________________________________________
Page 17
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
______________________________________________________________________________________ 17
voltage-divider to FBC (Figure 1). Resistor values can
be calculated with the following equation:
R8 = R9 ✕[(V
OUTC
/ V
FBC
) - 1]
where V
FBC
= +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, RCS(Figure 1). The
value of RCScan be calculated with the following equation:
R
CS
= V
CLM
/ (1.3 ✕ I
OUT
)
where V
CLM
= 80mV is the current-sense threshold,
and I
OUT
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 RCSas close to the MAX1774 as possible. The two traces should have matching length and
width, be as far as possible from noisy switching sig-
PGNDC
FBC
LXC
INC
FBM
CS-
CS+
PGND
ACI
PDRV
NDRV
CVH
MAIN
C5
1µF
P2
N1
L1
5µH
C
MAIN
47µF
C6
10µF
R
CS
CORE
C
CORE
22µF
R10
R11
R8
R9
L2
5.4µH
C7
1µF
R5
1MΩ
R6
1MΩ
R7
1MΩ
GND
REF
CVL
LXB
BIN
IN
DBI
LBI
BACKUP
BATTERY
MAIN
BATTERY
V
IN_AC
R1
C1
10µF
C2
10µF
C4
0.22µF
C3
1µF
D2
L3
22µH
P1
R4
R2
R3
D1
BKOFF
BKUP
LBO
ACO
SHDNC
SHDNM
MDRV
MAX1774
ON
OFF
ON
OFF
INS
NDS356AP
NSD03A10
2.7V
TO
20V
EP05
Q03L
LXB2 (QFN ONLY)
0.9V
TO
5.5V
1.0V
TO
5.5V
2.6V
TO
5.5V
FDS8928A
2.7V TO 28V
1MΩ
Figure 4. Typical Application Circuit (Cascaded)
Page 18
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
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 V
BL
is the low battery voltage, VBDis the dead
battery voltage, and VTH= +1.20V. Choose R3 to be
less than 250kΩ.
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
18 ______________________________________________________________________________________
Figure 5. Typical Application Circuit (with Recharge)
PGNDC
FBC
LXC
INC
FBM
CS-
CS+
PGND
PDRV
NDRV
CVH
MAIN
C5
L1
10µH
C
MAIN
47µF
R
CS
CORE
C
CORE
22µF
R10
R11
R8
R9
L2
C7
1µF
R5
1MΩ
R6
1MΩ
R7
1MΩ
GND
REF
CVL
LXB
BIN
IN
DBI
LBI
BACKUP
BATTERY
MAIN
BATTERY
R1
R13
C2
10µF
C3
C4
0.22µF
C1
10µF
P1
R4
R2
R3
R12
D1
BKOFF
BKUP
LBO
ACO
SHDNM
MDRV
MAX1774
L3
22µH
V
IN_AC
ACI
SHDNC
C6
10µF
P2
N1
ON
OFF
ON
OFF
1.0V
TO
5.5V
2.6V
TO
5.5V
FDS8928A
INS
1MΩ
2.7V
TO
20V
2.7V
TO
28V
NDS356AP
NSD03A10
0.9V
TO
5.5V
LXB2 (QFN ONLY)
D2
EP05
Q03L
Page 19
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
______________________________________________________________________________________ 19
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, L
MIN
:
L
(MIN)
= (VIN- V
OUT
) ✕(t
ON(MIN)
/ l
RIPPLE
)
where t
ONMIN
is typically 400ns, and l
RIPPLE
is the con-
tinuous conduction peak-to-peak l
RIPPLE
current.
In continuous conduction, l
RIPPLE
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).
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:
V
RIPPLE
≈ V
RIPPLE,ESR
+ V
RIPPLE,C
For tantalum capacitors, the ripple is determined mostly
by the ESR. Voltage ripple due to ESR is:
V
RIPPLE,ESR
≈ (R
ESR
) ✕ I
RIPPLE
For ceramic capacitors, the ripple is mostly due to the
capacitance. The ripple due to the capacitance is
approximately:
V
RIPPLE,C
≈ L I
RIPPLE
2
C
OUTVOUT
where V
OUT
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 (R
DS(ON)
), maxi-
mum drain-to-source voltage (V
DS(MAX)
), maximum
gate-to-source voltage (V
GS(MAX)
), and minimum
threshold voltage (V
TH(MIN)
).
Chip Information
TRANSISTOR COUNT: 4545
PROCESS: BiCMOS
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
LXC
INS
LBO
ACO
INC
GND
ACI
FBC
CSCS+
FBM
REF
LBI
DBI
BKOFF
BIN
LXB
CVH
PDRV
IN
CVL
NDRV
PGND
PGNDC
MDRV
BKUP
SHDNC
SHDNM
28 QSOP
TOP VIEW
MAX1774
Pin Configurations (continued)
Page 20
MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
20 ______________________________________________________________________________________
Package Information
QFN 28, 32,44, 48L.EPS
Page 21
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
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
Package Information (continued)
QSOP.EPS
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