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19-0653; Rev 0; 11/06
KIT ATION EVALU ABLE AVAIL
Complete Backup Management IC for NiMH Batteries
General Description Features
o o o o o o o o o o Charges 1- or 2-Cell NiMH Backup Batteries Programmable Charge Current DC Trickle Charge Mode for Maximum Cell Life Deep-Recovery Charge Restores Cells < 1V Programmable Charge Timer Programmable Charge-Voltage Limit and Battery UVLO Reverse Current Blocking on BATT, LDO, and Boost--No Diodes Needed No Battery Drain When Off (< 50nA) Thermistor Sensing Disables Standard Charge Battery Restart Charge Threshold Prevents Overcharge
MAX8672
The MAX8672 complete power manager for backup batteries in smart devices offers feature-programmable battery charging and main-system backup. The device includes a charger for 1- or 2-cell NiMH backup batteries. A low-quiescent current synchronous-rectified boost converter and LDO supply up to 20mA during system backup. The BST output is internally set to regulate at 3.05V. The MAX8672 LDO is powered from the boost converter output and is adjustable from 1.5V to 3.05V. The MAX8672 features programmable charge current, undervoltage lockout (UVLO), and maximum cell voltage. Charging is controlled by both a timer and thermistor monitor. Battery UVLO prevents excessive battery discharge and keeps inactive-battery drain current below 50nA. In addition, both LDO and boost converter outputs block reverse current so that diodes are not needed when connecting these outputs directly to system supplies. The MAX8672 requires that a valid system supply be present before system backup operation can occur. The MAX8672 is available in a 14-pin, 3mm x 3mm TDFN package and is rated for -40C to +85C operation.
Ordering Information
PART MAX8672ETD+T PIN-PACKAGE 14 TDFN-14 (3mm x 3mm) PKG CODE T14334+2
The MAX8672 operates in the -40C to +85C extended operating temperature range. +Denotes lead-free package.
Applications
PDA, Palmtop, and Wireless Handhelds Smart Cell Phones
Typical Operating Circuit
CT THRM
MAX8672
Pin Configuration
THRM CHGV
2.7V TO 5.5V INPUT
IN
CHGV DR GND LDO
TOP VIEW
CHGI TRKI
BST 10
FBL
14
13
12
11
LX 9
8 FBL BATT
MAX8672
BACKUP BATTERY CONNECTION UV
LDO
1.75V
1 CT
2 CHGI
3 TRKI
4 BATT
5 IN
6 DR
7 UV GND LX
BST
3.05V
TDFN (3mm x 3mm)
________________________________________________________________ Maxim Integrated Products
1
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.
Complete Backup Management IC for NiMH Batteries MAX8672
ABSOLUTE MAXIMUM RATINGS
IN, BATT, BST, LDO, UV to GND ..........................-0.3V to +6.0V FBL to GND ...............................................-0.3V to (VBST + 0.3V) CT, CHGI, TRKI, CHGV, THRM, DR to GND ...............................................-0.3V to (VIN + 0.3V) ILX ..................................................................................0.9ARMS Continuous Power Dissipation (TA = +70C) 14-Pin, 3mm x 3mm TDFN (derate 18.2 mW/C above +70C) .........................1454.5mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = 3.6V, TA = -40C to +85C, CCT = 0.1F, unless otherwise noted.) (Note 1)
PARAMETER IN Voltage Range IN Undervoltage Lockout Threshold IN Supply Current Internal Load Current on BST (Note 2) BATT Quiescent Supply Current--Backup Mode BATT Quiescent Supply Current--Charging BATT Leakage Current to IN Total BATT Battery Leakage Current During UVLO (BATT, LX, and DR Leakage) CHARGER AND BATTERY 1mA IBATT(CHG) 20mA, VIN - VBATT > 400mV CHGI Current-Limit Accuracy IBATT(CHG) = 1mA, VIN VBATT > 400mV 0.1mA IBATT(CHG) 1mA, VIN - VBATT > 400mV CHGI Bias Voltage CHGI Resistor Range IBATT(TRK) = 1mA TRKI Current-Limit Accuracy DC Trickle-Current Programming Range Charge-Current Programming Range TRKI Bias Voltage TRKI Resistor Range Charger Dropout Voltage VIN - VBATT where IBATT(CHG) falls by 10% of initial value; VIN = 3.6V, IBATT(CHG) = 20mA 100 250 IBATT(TRK) = 0.1mA IBATT(TRK) IBATT(CHG) TA = -40oC to +85oC TA = 0oC to +85oC TA = -40 C to +85 C
o o
CONDITIONS VIN rising, hysteresis = 100mV (typ) VBATT > VBATT(CHG) VIN = 3.3V VBST = 3.3V, no BST or LDO load, boost and LDO on VIN = 0V TA = -40C to +50C TA = -40C to +85C
MIN 2.7 2.20
TYP 2.45 40 117 91 91 3 3 0.01 0.07 5 50
MAX 5.5 2.60 100 170 125 150
UNITS V V A A
VBATT = 1.55V, VBST = 3.3V, VIN = 0V, VBATT(CHG) = 1.5V VBATT = 1.55V, VBST = 3.3V, VIN = 3.6V VBATT = 3.0V, VIN = 0V VBATT = 0 to 3.0V TA = +25oC TA = +85oC TA = -40C to +50C TA = +85oC
A A 0.1 50 A nA
TA = -40oC to +85oC TA = 0oC to +85oC TA = -40 C to +85 C TA = -40oC to +85oC
o o
-10 -10 -15 -20 600 5 -10 -10 -15 0.1 0.1 600
+10 +10 +15 +20 mV k % mA mA mV 1000 k mV %
1000 +10 +10 +15 1 20
2
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Complete Backup Management IC for NiMH Batteries
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, TA = -40C to +85C, CCT = 0.1F, unless otherwise noted.) (Note 1)
PARAMETER CHGV Output Current CHGV Resistor Range VBATT(CHG) Voltage-Limit Adjust Range VBATT(CHG) Voltage-Limit Accuracy VBATT(TRK) to VBATT(CHG) Ratio VBATT(RSTRT) to VBATT(CHG) Ratio VBATT(DR) to VBATT(CHG) Ratio Measured at BATT TA = +25 C RCHGV = 28.7k TA = 0oC to +50oC TA = -30oC to +85oC Sets 1.41V when VCHGV = 1.5V, measured at BATT Sets 1.225V when VCHGV = 1.5V, measured at BATT Sets 1.00V when VCHGV = 1.5V, measured at BATT; this is the VBATT above which deep recovery (DR) turns off; the falling threshold is typically 50mV below this IDR = 0 to 10mA Does not include capacitor error TA = +25 C TA = -40oC to +85oC 2 44 -2 42.00 325.5 45 -1 2 TA rising TA falling TA falling TA rising VUV = 1V 49.9 0.8 RUV = 49.9k TA = 0oC to +50oC TA = -30oC to +85oC -2 -3.25 1.5 1.225 1.25 0.08 50 5 TA = +25oC TA = +85oC 3 15 50 43.71 48.15 342.0 302.0 4 215 3.5 +2 +3.25 3.05 1.275 20 0.2 100 A k V % 358.6 45.42 k
o o o o
MAX8672
CONDITIONS VCHGV = 1V
MIN 28.7 1.50 -1 -1.25 -2.25 0.926 0.799 0.653 0.775
TYP 13
MAX 57.4 3.00 +1 +1.25 +2.25
UNITS A k V % -- -- -- -- % % min
o o o
0.940 0.816 0.667 0.816 10
0.954 0.832 0.680 0.861 20 20 25 2000 46 0
VDR Output Voltage to VBATT(CHG) Ratio Measured at BATT; no load on DR DR Load Regulation Charge-Timer Accuracy Timer Adjust Range Thermistor Hot-Trip Point Thermistor Cold-Trip Point Thermistor Temperature Hysteresis Hot-Trip Thermistor Resistance Cold-Trip Thermistor Resistance UV Output Current UV Resistor Range UV Battery-Cutoff Programmable Range UV Battery-Cutoff Accuracy LDO LDO Output-Voltage Range FBL Regulation Voltage LDO Output Current LDO Load Regulation LDO Dropout Voltage LDO Dropout Resistance FBL Input Bias Current Using external resistors, no load VBST = 3.3V, VLDO = 3.05V (Note 3) VBST = 3.3V, VLDO = 3.05V, ILDO = 1mA to 20mA VLDO = 2.5V, ILDO = 10mA VLDO = 2.5V VFBL = 1.25V
CHGI timer period, CCT = 0.047F = 8h (480min) RTHERM = 100k at TA = +25 C, TA rising RTHERM = 100k at TA = +25 C, TA falling
C C C
V V mA %/mA mV nA
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Complete Backup Management IC for NiMH Batteries MAX8672
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, TA = -40C to +85C, CCT = 0.1F, unless otherwise noted.) (Note 1)
PARAMETER BOOST CONVERTER BST Output Voltage Boost Output Current LX Current Limit n-Channel On-Resistance p-Channel On-Resistance n-Channel Maximum On-Time p-Channel Off-Current Threshold ILX = 200mA ILX = -200mA 3.5 5 1-cell input (Note 3) 400 500 0.4 0.7 5 20 2.989 3.05 3.111 20 600 1 2 6.5 35 V mA mA s mA CONDITIONS MIN TYP MAX UNITS
Note 1: Parameters are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design. Note 2: BATT current is higher due to boost ratio and efficiency. Note 3: Total load from both BST and LDO cannot exceed 20mA.
Typical Operating Characteristics
(VIN = 3.6V, Circuit of Figure 6, TA = +25C, unless otherwise noted.)
1-CELL CHARGE CURRENT (ICHG) vs. BATTERY VOLTAGE (VBATT)
MAX8672 toc01
2-CELL CHARGE CURRENT (ICHG) vs. BATTERY VOLTAGE (VBATT)
MAX8672 toc02
1-CELL CHARGE CURRENT (ICHG) vs. INPUT VOLTAGE (VIN)
ICHG = 20mA
MAX8672 toc03
25 ICHG = 20mA 20 CHARGE CURRENT (mA) VBST = 3.3V, VBATT(CHG) = 1.5V, RDR = 274, VBATT RISING
25 ICHG = 20mA 20 CHARGE CURRENT (mA) VBST = 3.3V, VBATT(CHG) = 3V, RDR = 549, VBATT RISING
25 20 CHARGE CURRENT (mA) 15 10 5 0 VBATT = 1.2V, RCHGV = 28.7k, VBST = 3.3V
15
15
10 ICHG = 3mA 5 ICHG = 200A 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 BATTERY VOLTAGE (V)
10 ICHG = 3mA
ICHG = 3mA
5 ICHG = 200A 0 0.5 1.0 1.5
0
0
-5 2.0 2.5 3.0 3.5 0
ICHG = 200A 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
BATTERY VOLTAGE (V)
2-CELL CHARGE CURRENT (ICHG) vs. INPUT VOLTAGE (VIN)
MAX8672 toc04
CHARGE CURRENT (ICHG) vs. TEMPERATURE
MAX8672 toc05
BATTERY-CHARGE PROFILE (2 NiMH CELLS)
MAX8672 toc06
25 20 CHARGE CURRENT (mA) 15 10 5 0 -5 0 VBATT = 2.4V, RCHGV = 57.6k, VBST = 3.3V
ICHG = 20mA
2.15
3.5 3.0 2.5 VBATT (V) 2.0 1.5 1.0 CHARGE TERMINATION AT VBATT(CHG) TRICKLE CHARGE STARTS AT VBATT(TRK)
CHARGE CURRENT (mA)
2.10
2.05
ICHG = 3mA
2.00 VBATT = 2.4V, VBST = 3.3V, RCHGI = 49.9k -40 -15 35 10 TEMPERATURE (C) 60 85
0.5 0 0
ICHG = 200A 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
1.95
VIN = 4V, VBST = 3.3V, R1 = 402k, R2 = 24.9k, R3 = 165k, R4 = R5 = 124k, R6 = 56k, R7 = OPEN, R8 = SHORT, R9 = 110k, C1 = 0.047F 60 120 180 240 300 360 420 TIME (min) 480
4
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Complete Backup Management IC for NiMH Batteries
Typical Operating Characteristics (continued)
(VIN = 3.6V, Circuit of Figure 6, TA = +25C, unless otherwise noted.)
BST EFFICIENCY vs. BST LOAD CURRENT
MAX8672 toc07
MAX8672
BATT INPUT CURRENT (WHILE BOOSTING) vs. BATT VOLTAGE
MAX8672 toc08
BST OUTPUT VOLTAGE vs. BST LOAD CURRENT
VIN = NOT CONNECTED BST OUTPUT VOLTAGE (V) 3.1
MAX8672 toc09
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 VIN = NOT CONNECTED 1 10 VBATT = 2.4V VBATT = 1.2V
500 450 400 BATT CURRENT (A) 350 300 250 200 150 100 50 0 ILDO ,IBST = 0A, VIN = NOT CONNECTED, BACKUP MODE 0.8 1.2 1.6 2.0 2.4 2.8
3.2
3.0 VBATT = 1.2V 2.9
VBATT = 2.4V
2.8 3.2 0 30 60 90 120 150 180 BST LOAD CURRENT (mA)
100
BST LOAD CURRENT (mA)
BATT VOLTAGE (V)
DR CURRENT (IDR) vs. BATTERY VOLTAGE (VBATT)
MAX8672 toc10
BATT INPUT LEAKAGE CURRENT DURING UVLO vs. TEMPERATURE
BATT INPUT LEAKAGE CURRENT (nA) 17.5 15.0 CT FREQUENCY (Hz) 12.5 10.0 7.5 5.0 2.5 0.0 VIN = VBST = VLDO = 0V VBATT = 0.9V RCHGV = 28.7k -40 -15 10 35 60 85 TEMPERATURE (C) 15.5 15.0 -40 17.0 16.5 16.0
MAX8672 toc11
CT FREQUENCY vs. TEMPERATURE
MAX8672 toc12
25
20.0
18.0 17.5
20 DR CURRENT (mA)
15 VBATT(CHG) = 3.0V 10
5 VBATT(CHG) = 1.5V 0 0.5 1.0 15 2.0 RDR = 100 2.5 3.0
0
CCT = 0.047F, X7R, 10% -15 10 35 60 85 TEMPERATURE (C)
BATTERY VOLTAGE (V)
MAX BATT CHARGE VOLTAGE vs. TEMPERATURE (1-CELL NiMH)
MAX8672 toc13
LIGHT-LOAD SWITCHING WAVEFORMS
MAX8672 toc14
1.55 MAX BATT CHARGE VOLTAGE (V)
1.53
VBST 50mV/div VLX 1V/div
3.05V
1.51
1.49
0V
1.47 RCHGV = 28.7k 1.45 -40 -15 10 35 60 85 TEMPERATURE (C) ILX 200mA/div 10s/div IBST = 2mA 0A
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5
Complete Backup Management IC for NiMH Batteries MAX8672
Typical Operating Characteristics (continued)
(VIN = 3.6V, Circuit of Figure 6, TA = +25C, unless otherwise noted.)
HEAVY-LOAD SWITCHING WAVEFORMS
MAX8672 toc15
BST LOAD TRANSIENT
MAX8672 toc16
VBST 50mV/div VLX 1V/div
3.05V
IBST 20mA/div
0A
0V 0V ILX 200mA/div 10s/div VBST 20mV/div AC-COUPLED 0A 200s/div
IBST = 20mA
VBATT = 2.4V
BST OUTPUT VOLTAGE vs. LDO LOAD CURRENT
VIN = NOT CONNECTED BST OUTPUT VOLTAGE (V) 3.1
MAX8672 toc17
LDO LOAD TRANSIENT
MAX8672 toc18
3.2
ILDO 20mA/div
0A
3.0 VBATT = 2.4V VBATT = 1.2V 2.9 VLDO 10mV/div AC-COUPLED VBATT = 2.4V 0 30 60 90 120 150 180 200s/div 0V
2.8 LDO LOAD CURRENT (mA)
BST RESPONSE TO LDO LOAD TRANSIENT
MAX8672 toc19
BATTERY RIPPLE (VBATT_P-P) vs. TEMPERATURE
MAX8672 toc20
300 250 CBATT = 10F VBATT_P-P (mV)
ILDO 20mA/div 0A
200 150 100
VBST 20mV/div AC-COUPLED VBATT = 2.4V 200s/div
0V 50 0
CBATT = 47F
-20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 TEMPERATURE (C)
6
_______________________________________________________________________________________
Complete Backup Management IC for NiMH Batteries
Pin Description
PIN 1 2 3 4 5 NAME CT CHGI TRKI BATT IN FUNCTION Programming Input for Charge Timer. Connect a capacitor from CT to GND to program the charge timer (range: 2min to 2000min, nominally programmed to 8h = 480min with CCT = 0.047F). Programming Input for the Standard Charge-Current Rate. Connect a resistor (RCHGI) from CHGI to GND to program the standard charge current from 100A to 20mA. Programming Input for the DC Trickle-Charge Rate. Connect a resistor from TRKI to GND to program the trickle-charge current. Backup-Battery Connection. The backup battery charges from IN but does not allow reverse current to IN when VIN < VBATT. BATT input current is less than 0.1A when VBATT is below the UV threshold. Power Input. Range is 2.7V to 5.5V. Programming Input for Deep-Recovery Threshold. The DR output adds charge current when VBATT is below the VBATT(DR) threshold (and THRM is valid) by biasing an external resistor connected from DR to BATT. The DR output voltage, VDR, is 0.816 times the VBATT(CHG) limit set by VCHGV (VDR = 1.224V for a 1.5V VBATT(CHG)). The DR current is sourced in addition to the standard charge current set by RCHGI. Programming Input for the BATT Undervoltage Lockout (UVLO), VBATT(UV). The UVLO threshold is programmed by connecting a resistor from UV to GND. The backup LDO and boost converter cannot start after UVLO occurs, or on power-up, until a valid VIN and VBST are applied. VBATT(UV) is programmable from VBATT = 0.8V to 3.5V. An open circuit at UV disables the boost and LDO and interrupts battery drain. UVLO also latches off backup circuitry to minimize battery drain. Ground Boost Converter Switch Node. Connect the boost inductor from LX to BATT. Boost Converter Output. BST has reverse current blocking when VBST is higher than VIN or VBATT. The MAX8672 operates with VBST down to 2.35V. The BST output is factory preset for 3.05V for use with 3.3V systems. Other voltages are available on request. LDO Output. Programmable from 1.5V to 3.05V. LDO has reverse current blocking. Programming Input for the LDO Output Voltage. Connect FBL to the center of a resistor-divider connected between LDO and GND. The FBL threshold is 1.25V. Programming Input for the Charge Voltage Limit (VBATT(CHG)). Also programs the trickle threshold (VBATT(TRK)), standard charge-restart voltage (VBATT(RSTRT)), DR threshold (VBATT(DR)), and the DR output voltage (VDR). For NiMH, program 1.5V VBATT(CHG) per cell, so that the max possible voltage is 1.55V per cell with tolerances. VBATT(CHG) is programmable from 1.5V to 3.0V by connecting a resistor from CHGV to GND. When the battery voltage rises to VBATT(CHG), standard charging stops. When the battery voltage falls to VBATT(TRK), trickle charge begins. Standard charge does not resume until the battery voltage falls to VBATT(RSTRT). 14 THRM External Thermistor Monitor Connection. Connect an NTC (100k at TA = +25C) thermistor for -1C and +45C charging cutoff. Only trickle charging is allowed outside the temperature limits. These temperature thresholds are programmable by adding series and parallel resistors to the external thermistor. See Table 1. Exposed Pad. Connect to GND but do not rely on EP for ground functions. This pad is internally connected to ground through a soft connect, meaning there is no internal metal or bond wire physically connecting the exposed pad to the GND pin. Connecting the exposed pad to ground does not remove the requirement for a good ground connection to the appropriate pins. For good thermal dissipation, the exposed pad must be soldered to the power ground plane.
MAX8672
6
DR
7
UV
8 9 10 11 12
GND LX BST LDO FBL
13
CHGV
--
EP
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7
Complete Backup Management IC for NiMH Batteries MAX8672
100k REF
Detailed Description
THRM
OK HOT OK COLD VCHG COMP TIMER + LOGIC VTRK COMP 2X CHGV
CT
IN
VRSTRT COMP
DR
A=2
CHGI TRKI VBATT / 2
VDR COMP
The MAX8672 is a compact IC for managing backupbattery charging and utilization in PDAs and other smart handheld devices. The IC contains three major blocks: a charger for 1 or 2 NiMH coin cells; a small, very-lowquiescent current step-up DC-DC converter that generates a boosted backup supply; and an LDO that can supply a 2nd backup voltage to an additional system block (typically low-voltage RAM). The MAX8672 does not have a logic control signal for activating backup. The main system supplies are directly connected to the BST and LDO outputs, where LDO and BST are programmed to regulate just below system supply voltages. When system supply voltages exceed the programmed BST and LDO output voltage, BST and LDO are pulled up by the system supplies and do not sink current (BST sinks 80A for chip operation). When the system supplies fall below the programmed output voltage, BST and LDO operate to maintain system voltages at the programmed values. The LDO and boost converter do not operate any differently in the system's running (and charging) state than they do in the backup state. The LDO and BST error amplifiers constantly monitor their outputs in both cases. The MAX8672 has three states: * System Active/Charging. With a valid VIN (greater than 2.7V and also greater than VBATT), and a valid V BST (greater than 2.35V), the battery charges. LDO and BST are active and available for system backup. Charging and system backup are independent functions. * Backup. When the system supply voltages have fallen below the programmed output voltage, BST and LDO maintain their output voltages and are sourced by the battery. Under these conditions, battery charging has ceased, but this is not a requirement for the backup state. Off. When the battery voltage has fallen below the UVLO threshold (VBATT(UV)) and VIN is not valid, the IC turns off and all outputs are latched off. If VBATT recovers to above VBATT(UV), charging does not resume until both a valid VIN and VBST are present. Negligible battery current (less than 50nA leakage) is drawn in this state.
A=1 BATT R R
LDO UV OFF
FBL
MAX8672
3.05V
LDO HYST*: 0.85V HYSTERESIS, COMPARATOR HI AT 3.05V, LO AT 2.2V
HYST*
AND
AND HI
*
AND LO LX LOW IQ BOOST CONVERTER PFM BST
Figure 1. MAX8672 Functional Block Diagram
8
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Complete Backup Management IC for NiMH Batteries
Charger
The MAX8672 charger is a comparator-controlled current source with both current and voltage limits programmed by external resistors. Typical charge profiles for a 1-cell NiMH battery are shown in Figure 2 and explained below. When power is applied at IN and BST, the MAX8672 charges the battery at the standard charge current programmed by a resistor connected between CHGI and GND. The MAX8672 remains in standard charge until the charge timer (programmed by CCT) times out, the battery rises to the VBATT(CHG) limit, or the charge is interrupted by a temperature-range violation. If standard charge is terminated by the charge timer, trickle charge mode begins and continues without timing until the VBATT(CHG)
CHARGE TIMER EXPIRES 1.5V VBATT(CHG) NO CHARGE TRICKLE
limit is achieved. Once standard charge or trickle charge is terminated by the VBATT(CHG) limit, charging ceases. Subsequently, if VBATT falls to the VBATT(TRK) threshold, trickle charge is activated. VBATT then rises and the charging cycle continues. The charger does not enter standard charge again until the battery falls to the VBATT(RSTRT) threshold. When the VBATT(RSTRT) threshold is reached, standard charge begins and the charge timer is reset. Standard charge is also interrupted if the external thermistor temperature sensed at THRM is out of range. When THRM senses a too-hot or too-cold condition during standard charge, the timer pauses and the charger enters trickle charge.
MAX8672
NO CHARGE
1.41V VBATT(TRK) STANDARD CHARGE
1.225V VBATT(RSTRT) 1.5V VBATT(CHG)
NO CHARGE
TRICKLE
1.41V VBATT(TRK)
NO CHARGE TRICKLE
STANDARD CHARGE
1.225V VBATT(RSTRT)
Figure 2. Typical Charge-Current Profiles for 1-Cell Battery
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9
Complete Backup Management IC for NiMH Batteries
Additionally, if VIN is interrupted during standard charge, and the battery voltage is greater than VBATT(RSTRT), the timer pauses until power is reapplied. If the battery voltage falls below VBATT(RSTRT), the timer resets. See the charger state diagram in Figure 3 for more details on charger operation. Trickle charge occurs whenever standard charge is interrupted by timeout, when VBATT falls to VBATT(TRK), or when THRM senses an out-of-temperature-range condition. Trickle charge has the same voltage limit as standard charge and cannot drive the battery above VBATT(CHG). A valid voltage is required on both IN and BST for standard and trickle charging. Once charging begins, if VIN becomes invalid, charging stops, but the timer is paused since the backup circuitry is supplying BST. If VBST falls below 2.2V, the timer resets. If the thermistor hot- or cold-temperature threshold is violated, the charge timer pauses and only trickle charging is allowed. When THRM recovers, the MAX8672 goes to RUN instead of reentering the charge mode. This is done to reevaluate the battery state when the temperature returns to the normal operating range. The charge timer is not reset when returning to the RUN state.
MAX8672
VBST NOT OK VBST NOT OK OFF (RESET TIMER) ANY STATE
VBST OK VIN NOT OK AND VBST OK STAND BY (TIMER PAUSED)
VIN NOT OK AND VBST OK
ANY STATE EXCEPT TRICKLE CHARGE AND NO CHARGE
VIN OK VBATT > VBATT(RSTRT) T > COLD AND T < HOT
VIN NOT OK AND VBST OK (2-CELL, VCHG > 2.1V)
RESET TIMER
RUN
VBATT < VBATT(DR) - 50mV RESET TIMER COMPLETE VBATT > VBATT(DR)
COLD/HOT TRICKLE (TIMER PAUSED) VIN NOT OK AND VBST OK (1-CELL, VCHG < 2.1V)
VBATT < VBATT(RSTRT)
STANDARD CHARGE
DEEPRECOVERY CHARGE
T < COLD OR T > HOT
VBATT < VBATT(DR) - 50mV TIMER COMPLETE T < COLD AND T > HOT NO CHARGE AND FORCE TIMER COMPLETE
VBATT > VBATT(CHG) VBATT < VBATT(TRK) TRICKLE CHARGE VBATT > VBATT(CHG)
VBATT > VBATT(CHG)
Figure 3. Charger State Diagram
10 ______________________________________________________________________________________
Complete Backup Management IC for NiMH Batteries
Charger Voltage and Standard Charge-Current Limits The MAX8672 charger is a comparator-controlled current source with both current and voltage limits programmed by external resistors. The maximum battery charge-voltage limit (VBATT(CHG)) is programmed by connecting a resistor (RCHGV) from CHGV to GND (Figure 4). The range for the charging voltage limit is 1.5V to 3.0V. For NiMH batteries, VBATT(CHG) is typically selected for a 1.5V max charge per cell. After selecting VBATT(CHG) for the intended application, the required RCHGV is determined by the following equation:
RCHGV = VBATT(CHG) 52265 x 10-6 . or when THRM senses an out-of-temperature-range condition. Trickle charge has the same voltage limit as standard charge. The trickle current is programmed from 100A to 1mA by connecting a resistor (R TRKI) from TRKI to GND (Figure 4). After selecting the battery trickle charge current (IBATT(TRK)) for the application, RTRKI is determined by the following equation: RTRKI (k) = 100 IBATT(TRK)(mA)
MAX8672
R8 CT C1 (CCT) THRM TH1 100k AT +25C CHGV DR C6 (CDR) R1 (RDR) R2 (RCHGI) R3 (RTRKI) CHGI TRKI FBL R5 BATT BACKUP BATTERY C3 (CBATT) UV L1 R9 (RUV) GND LX BST C4 (CBST) LDO C5 (CLDO) R6 (RCHGV) R4
(Note that the voltage at CHGV is VBATT(CHG) / 4.) The other voltage thresholds associated with the charging cycle (Figure 2) are dependent upon the selection of VBATT(CHG) as follows: Falling battery threshold to begin trickle charge (VBATT(TRK)): VBATT(TRK) = 0.94 x VBATT(CHG) Falling battery threshold to restart standard charge (VBATT(RSTRT)): VBATT(RSTRT) = 0.816 x VBATT(CHG) Rising battery threshold to exit deep-recovery charge (VBATT(DR)): VBATT(DR) = 0.667 x VBATT(CHG) Standard charging of the battery occurs when the MAX8672 is first turned on, or when the battery is discharged below the VBATT(RSTRT) threshold. Standard charge ceases when the VBATT(CHG) limit is reached. The standard charge current (I BATT(CHG) ) is programmed from 0.1mA to 20mA by connecting a resistor (RCHGI) from CHGI to GND (Figure 4). The valid range of RCHGI is 5k to 1M. Once the value of standard charge current (I BATT(CHG) ) has been chosen, the required RCHGI is determined by the following equation: RCHGI (k) = 100 IBATT(CHG)(mA)
MAX8672
2.7V TO 5.5V INPUT C2 (CIN) IN
Figure 4. External Component Diagram
Trickle Charge Trickle charge occurs whenever standard charge is interrupted by timeout, when VBATT falls to VBATT(TRK),
Deep-Recovery Charge The MAX8672 includes a circuit to bring up deep discharged NiMH cells. When power is first applied to IN, if the battery voltage is less than the battery deeprecovery threshold, VBATT(DR), DR connects an internally regulated voltage to an external resistor that sources extra current into the battery. The DR currentlimiting resistor is typically selected for a 0.5C charge rate when the cell voltage is 0V. When DR is on, both the standard charge current and the DR current charge the battery. When the cell voltage reaches VBATT(DR), DR current is turned off and standard charging begins.
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Complete Backup Management IC for NiMH Batteries MAX8672
DR charging is allowed only when the THRM temperature is within hot and cold limits. The rising battery-voltage threshold for DR (V BATT(DR) ) is given by the following equation: VBATT(DR) = 0.667 x VBATT(CHG) The DR output voltage is: VDR = 0.816 x VBATT(CHG) below the minimum tolerance for the main supply. When the main supply voltage drops below its specified level, the step-up converter begins regulating as long as the load is 20mA or less. The MAX8672 blocks reverse current flow if VBST is higher than VBATT. The MAX8672 typical application expects that a valid system voltage is connected to BST and IN before backup operations are required. The boost DC-DC converter is able to supply a system load (up to 20mA) when the main power source falls below the BST preset voltage, but the IC cannot start up the BST output with just the backup battery alone. BST must initially be powered by the external system in order for the boost converter to start. Then, if the system voltage falls below the BST preset voltage, the boost converter can supply the load. If necessary, this limitation can be overcome for some applications by connecting a diode from IN to BST, so that BST is immediately powered from IN. When VBATT is less than VBST, and VBST is not externally pulled above 3.05V by the main system supply, the boost converter runs as needed to maintain VBST at 3.05V. If, during normal active/charging mode operation, VBATT rises above the main system voltage that is connected to BST, current may flow from the battery to the main system supply, even though no backup operation is expected. For example, in a 2-cell system, if VBATT is 3.2V and the system supply is holding BST at 3.1V, then the backup battery drains into the system supply. The boost synchronous rectifier pMOS contains a body diode that is switched to prevent unwanted current flow (see the BATT-BST Current Flow section). Since the normal maximum charge limit (VBATT(CHG)) for 2 NiMH cells is usually set to 3.0V (for a 3.1V max), and a 3.3V system supply less a 5% tolerance is 3.135V, VBATT does not exceed VBST during normal system operation, resulting in no backup current flow. However, for other BATT or BST voltages where unwanted backup current flow may occur, it can be prevented by connecting a diode in series with the boost inductor to reduce the voltage at BST. The diode may be a Schottky or silicon signal diode, depending on how much voltage needs to be dropped.
Thermistor Monitor The thermistor monitor suspends standard charging (and pauses the standard charge timer) when the thermistor temperature moves above +45C or below -1C. The thermistor must be an NTC type with a nominal +25C resistance of 100k.
The temperature trip thresholds are adjusted by adding external resistors in series and in parallel with the thermistor. For the specified thermistor, the resistors values are shown in Table 1.
Table 1. Series/Parallel Resistors for Different Thermistor Thresholds ()
SERIES R (k) 0 7.5 13.7 18.7 18.8 22.7 23 PARALLEL R (M) None None None None 6.8 None 5.6 HOT TEMP (OC) 45 50 55 60 59.9 65 65 COLD TEMP (OC) -1 -0.6 -0.3 0 -1 0 -1 -5
8.6 1.7 50 Note: With 100k thermistors at +25C, = 3977.
Charge Timer The MAX8672 includes a charge timer that is programmable from 2min to 2000min. Timer duration is programmed by a capacitor, CCT, connected from CT to GND (Figure 4). The charge-timer duration (tCHG) is determined by the equation:
t CHG (minutes) = 10195 x CCT (F)
Boost DC-DC Converter
The MAX8672 contains a low-current synchronous-rectified boost converter that can supply up to 20mA. The boost converter's preset output voltage is 3.05V, intended for backing up a 3.3V supply. Preset output voltages can be obtained from the factory on request. Generally, the output voltage is programmed to be just
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Boost Output Capacitor Selection Choose output capacitors to supply output peak currents with acceptable voltage ripple. Low equivalent series resistance (ESR) capacitors are recommended. Ceramic capacitors have the lowest ESR, but low-ESR tantalum or polymer capacitors offer a good balance between cost and performance.
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Complete Backup Management IC for NiMH Batteries
Output-voltage ripple has two components: variations in the charge stored in the output capacitor (CBST) with each BST pulse, and the voltage drop across the capacitor's ESR due to the current flow into and out of the capacitor. The equations for approximating outputvoltage ripple are: VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) VRIPPLE(ESR) = IPEAK x RESR VRIPPLE(C) = 1 L 2 IPEAK 2 (VBST - VBATT ) x CBST
LDO
For backup designs that require two different backup voltages, the MAX8672 includes a small LDO, which is powered from BST. This LDO can supply up to 20mA. Generally, the output voltage is programmed to be just below the minimum tolerance for the main supply. When the main supply voltage drops below its specified level, the LDO begins regulating. The LDO output voltage is adjustable from 1.5V to 3.05V using external resistors (R4 and R5 in Figure 4). Since the FBL input bias current is 50nA (max), select feedback resistor R4 in the 100k to 1M range. After choosing R4, calculate R5 as follows: V R5 = R4 LDO - 1 VFBL where VFBL = 1.25V.
MAX8672
where IPEAK is the peak inductor current (see the Boost Inductor Selection section). Since ESR is usually very small in ceramic capacitors, the output ripple is typically dominated by VRIPPLE(C). Capacitance and ESR variation with temperature should be considered for best performance in applications with wide operating-temperature ranges.
Backup-Battery Bypass Capacitor Selection
The MAX8672 boost converter draws 500mA short-term inductor-charging current peaks from the battery when the boost converter operates. Small coin cells that are commonly used for backup often exhibit high output impedance that varies over temperature. For this reason, the backup battery must be bypassed with a highquality ceramic capacitor with X7R, X5R, or better dielectric (CBATT, Figure 4). Typical values are between 10F and 47F. Note that high battery ripple can prematurely trigger the UVLO comparator and shut down the boost circuit before the battery is fully discharged. If this is a concern with the selected battery, the UV threshold may be lowered, in addition to using a larger battery bypass capacitance, to accommodate the short-term battery-voltage dip due to ripple. See the Battery Ripple vs. Temperature graph in the Typical Operating Characteristics section.
Boost Inductor Selection The control scheme of the MAX8672 permits flexibility in choosing an inductor. A 4.7H inductor performs well in most applications. For maximum output current, choose the inductor value so that the controller reaches the current limit before the maximum on-time is reached:
L< VBATT x t ON(MAX) ILIM
where tON(MAX) is typically 5s, and the current limit (ILIM) is typically 500mA (see the Electrical Characteristics). For larger inductor values, determine the peak inductor current (IPEAK) by: IPEAK = VBATT x t ON(MAX) L
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Complete Backup Management IC for NiMH Batteries
BACKUP READY* (BOOST AND LDO ENABLED)
VBATT < VBATT(UV)
VIN > 2.45V AND VBST > 3.05V AND VBATT > VBATT(UV)
BATT UVLO (BOOST AND LDO OFF)
Note: In order for BATT current to remain below 50nA during BATT UVLO, VBST must fall below 0.5V. If VBST is held up by another source during UVLO, or if VBST is higher than 0.5V, BATT input current during BATT UVLO is typically 500nA. Typically, VBST falls to 0V in most situations. If minimum battery drain during BATT UVLO is critical, then an external pulldown resistor connected between BST and GND may be needed to discharge the BST output. The 500nA BATT drain during UVLO is necessary when VBST is > 0.5V because a comparator must be kept active in order to detect the higher of VBATT or VBST. This comparator switches the body diode of the internal FET connecting these outputs to ensure that current flow is blocked. When VBST falls to approximately 0.5V, the comparator is shut off, and the FET body is connected to block current flowing from BATT to BST.
MAX8672
*NOTE: BACKUP READY DOES NOT MEAN THAT THE BOOST AND LDO ARE OPERATING. WHEN THE BST AND LDO OUTPUTS ARE ENABLED; THEY STILL ONLY OPERATE IF NEEDED WHEN THE SYSTEM FAILS TO HOLD UP THE SUPPLIES.
BATT-BST Current Flow
The MAX8672 synchronous rectifier pMOS contains an internal body diode connected between BATT and BST. This diode switches to prevent undesired current flow between these pins. Upon startup, the body diode points to the greater of VBATT or VBST, until VBST rises above 3.05V (at least once). Then the body diode switches to point to BST. The body diode points from BATT to BST until V BST falls below 2.2V. When this occurs, the body diode switches to point to the greater of VBATT or VBST. If VBATT exceeds VBST by a few hundred millivolts or more, the body diode is forward biased and current flows from BATT to BST. This is the typical case for a boost converter when the input exceeds the output. When backing up, this typically is not a problem since it is expected that battery current powers the system. When not in backup mode (system power is up and is pulling VBST over 3.05V), current can flow from BATT to BST if VBATT exceeds VBST by enough to forward bias the diode. With two NiMH cells, VBATT charges to 3.0V nominal (3.1V max), so with VBST pulled to more than 3.05V by the system, there is not enough voltage difference to cause significant current to flow from BATT to BST.
Figure 5. Backup and BATT UVLO State Diagram
BATT Undervoltage Lockout
When the backup battery discharges to a programmed threshold, VBATT(UV), BATT UVLO is engaged. As a result, the MAX8672 backup functions (BST and LDO) shut down, and a small current (less than 50nA) is drawn from BATT. During BATT UVLO, charge functions still remain active to recharge the battery. Once BATT UVLO occurs, the backup boost converter and LDO do not reactivate until VBST rises above 3.05V and VIN rises above 2.45V (typ). Even if BATT recovers, the backup functions do not activate until a valid VIN and VBST have been present. See the Backup and BATT UVLO State Diagram (Figure 5). The BATT UVLO threshold (VBATT(UV)) is programmed by connecting a resistor (RUV) from UV to GND (Figure 4). For NiMH cells, the UVLO threshold is typically programmed to 0.8V per cell. Once the UVLO threshold value is determined, RUV is calculated from the following equation: VBATT(UV) RUV = 16 x 10-6
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Complete Backup Management IC for NiMH Batteries
Applications Information
Typical Application Circuit
Figure 6 displays the MAX8672 typical application circuit for 2-cell NiMH applications. Corresponding to the requirements for 2-cell NiMH batteries, maximum charge voltage is programmed for 3.0V and the UVLO threshold is set to 1.6V. The LDO output voltage is 1.75V. Standard charge provides 2mA of standard charge current, while trickle charge is programmed to provide 500A of trickle charge current. A 7.5k resistor is connected in series with the thermistor to program a hot temperature threshold of +50C and a cold temperature threshold of -0.6C.
CT C1 0.047F 2.7V TO 5.5V INPUT C2 0.1F DR C6 0.22F R1 100 R2 50k R3 200k CHGI TRKI CHGV R6 57.6k R4 250k FBL BATT 2-CELL NiMH L1 4.7H C3 47F LDO UV R9 100k GND LX C5 0.47F BST C4 22F 3.05V R5 100k 1.75V IN THRM TH1 100k AT +25C
MAX8672
R8 7.5k
MAX8672
Layout Guidelines
Careful PCB layout is important for minimizing ground bounce and noise. Ensure that C2 (IN input capacitor), C3 (BATT input capacitor), C4 (BST bypass capacitor), and C5 (LDO output capacitor) are placed as close as possible to the IC. Avoid using vias to connect C3 or C4 to their respective pins or GND. C3 and C4 grounds should be located next to each other, and this connection can then be used as the star ground point. All other grounds should connect to the star ground. Connect EP to the bottom layer ground plane, and then connect the ground plane to the star ground. Vias on the inductor path are acceptable, if necessary. IN, BATT, BST, and LDO traces should be as wide as possible to minimize inductance. Refer to the MAX8672 evaluation kit for a PCB layout example.
Figure 6. Typical Application Circuit for the MAX8672 Using a 2-Cell NiMH
Chip Information
PROCESS: BiCMOS
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Complete Backup Management IC for NiMH Batteries MAX8672
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
6, 8, &10L, DFN THIN.EPS
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
21-0137
H
1 2
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Complete Backup Management IC for NiMH Batteries
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
MAX8672
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
PACKAGE VARIATIONS PKG. CODE T633-1 T633-2 T833-1 T833-2 T833-3 T1033-1 T1033-2 T1433-1 T1433-2 N 6 6 8 8 8 10 10 14 14 D2 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.700.10 1.700.10 E2 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 e 0.95 BSC 0.95 BSC 0.65 BSC 0.65 BSC 0.65 BSC 0.50 BSC 0.50 BSC 0.40 BSC 0.40 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEA MO229 / WEEC MO229 / WEEC MO229 / WEEC MO229 / WEED-3 MO229 / WEED-3 ------b 0.400.05 0.400.05 0.300.05 0.300.05 0.300.05 0.250.05 0.250.05 0.200.05 0.200.05 [(N/2)-1] x e 1.90 REF 1.90 REF 1.95 REF 1.95 REF 1.95 REF 2.00 REF 2.00 REF 2.40 REF 2.40 REF
0.25 MIN. 0.20 REF.
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
H
2 2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 17
(c) 2006 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

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