Maxim MAX1641D Adjustable-output, switch-mode current sources with synchronous rectifier Datasheet

19-1245; Rev 0; 7/97
KIT
ATION
EVALU
LE
B
A
IL
A
AV
Adjustable-Output, Switch-Mode
Current Sources with Synchronous Rectifier
________________________Applications
Battery-Powered Equipment
Laptop, Notebook, and Palmtop Computers
____________________________Features
♦ 95% Efficiency
♦ +5.5V to +26V Input Supply Range
♦ 2V to 24V Adjustable-Output Voltage Range
♦ 100% Maximum Duty Cycle (Low Dropout)
♦ Up to 500kHz PWM Operation
♦ Optional Synchronous Rectifier
♦ 16-Pin QSOP Package
♦ Current-Sense Accuracy: 2% (MAX1641)
5.3% (MAX1640)
______________Ordering Information
TEMP. RANGE
PART
MAX1640C/D
0°C to +70°C
MAX1640EEE
MAX1641C/D
MAX1641EEE
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
PIN-PACKAGE
Dice*
16 QSOP
Dice*
16 QSOP
*Dice are specified at TA = +25°C, DC parameters only.
Handy Terminals
Portable Consumer Products
__________Typical Operating Circuit
Cordless Phones
Cellular Phones
VIN = +5.5V TO +26V
PCS Phones
Backup Battery Charger
IN
D0
LDOH
PDRV
P
D1
__________________Pin Configuration
TOFF
NDRV
REF
PGND
RTOFF
TOP VIEW
LDOL 1
16 IN
TOFF 2
15 LDOH
D1 3
14 PDRV
D0 4
CC 5
MAX1640
MAX1641
SET
CS-
OUT
13 NDRV
MAX1640
12 PGND
REF 6
11 CS+
SET 7
10 CS9
TERM 8
CS+
CC
GND
TERM
LDOL
GND
QSOP
________________________________________________________________ Maxim Integrated Products
1
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For small orders, phone 408-737-7600 ext. 3468.
MAX1640/MAX1641
_______________General Description
The MAX1640/MAX1641 CMOS, adjustable-output,
switch-mode current sources operate from a +5.5V to
+26V input, and are ideal for microprocessor-controlled
battery chargers. Charging current, maximum output
voltage, and pulse-trickle charge are programmed with
external resistors. Programming the off-time modifies
the switching frequency, suppressing undesirable harmonics in noise-sensitive circuits. The MAX1640’s highside current sensing allows the load to connect directly
to ground, eliminating ground-potential errors. The
MAX1641 incorporates a low-side current sense.
The MAX1640/MAX1641 step-down pulse-width-modulation (PWM) controllers use an external P-channel
MOSFET switch and an optional, external N-channel
MOSFET synchronous rectifier for increased efficiency.
An internal low-dropout linear regulator provides power
for the internal reference and circuitry as well as the
gate drive for the N-channel synchronous rectifier.
The MAX1640/MAX1641 are available in space-saving,
16-pin narrow QSOP packages.
MAX1640/MAX1641
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
ABSOLUTE MAXIMUM RATINGS
IN to GND ...............................................................-0.3V to +28V
LDOH to IN ...............................................................+0.3V to -6V
LDOL to GND ...........................................................-0.3V to +6V
PDRV to GND .............................. (VLDOH - 0.3V) to (VIN + 0.3V)
NDRV to GND .........................................-0.3V to (VLDOL + 0.3V)
TOFF, REF, SET, TERM, CC to GND ......-0.3V to (VLDOL + 0.3V)
D0, D1 to GND .........................................................-0.3V to +6V
CS+, CS- to GND ...................................................-0.3V to +28V
PGND to GND.....................................................................±0.3V
Continuous Power Dissipation (TA = +70°C)
QSOP (derate 8.30mW/°C above +70°C) ................... 667mW
Operating Temperature Range
MAX164_EEE ...................................................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10sec) ............................ +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
(VIN = +12V, VOUT = 6V, Circuit of Figure 2, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Input Voltage Range
SYMBOL
CONDITIONS
VIN
MIN
TYP
5.5
MAX
UNITS
26
V
Linear-Regulator Output
Voltage, VIN Referenced
VLDOH
VIN = 5.5V to 26V, ILOAD = 0 to 20mA
VIN 5.5
VIN 5.0
VIN 4.5
V
Linear-Regulator Output
Voltage, Ground Referenced
VLDOL
VIN = 5.5V to 26V, ILOAD = 0 to 20mA
4.5
5.0
5.5
V
Full-Scale Current-Sense
Threshold
MAX1640
142
150
158
MAX1641
147
150
153
Quarter-Scale Current-Sense
Threshold
MAX1640
36
42
48
MAX1641
34
37.5
41
Current-Sense Line Regulation
VIN = VOUT + 0.5V to 26V
Output Current Compliance
VOUT = 2V to 24V
MAX1640
0.1
MAX1641
0.1
D0 or D1 = high
Quiescent VIN Supply Current
VREF
4
%/V
mA
µA
1
µA
4.20
4.35
V
1.96
2.00
2.04
V
4
10
mV
1
µA
12
Ω
IREF = 0 to 50µA
PFET and NFET drive
Off-Time Range
0.4
4.05
VSET Input Current
FET Drive Output Resistance
mV
%/V
500
D0 = D1 = low
VLDOL Undervoltage Lockout
Reference Load Regulation
2
D0 = D1 = low (off mode)
Output Current in Off Mode
Reference Voltage
0.03
mV
10
µs
Off-Time Accuracy
RTOFF = 62kΩ
1.7
1
2.2
2.7
µs
Pulse-Trickle Mode Duty-Cycle
Period
D0 = low, D1 = high, RTOFF = 100kΩ
27
33
40
ms
Pulse-Trickle Mode Duty Cycle
(Note 1)
D0 = low, D1 = high, RTOFF = 100kΩ
12.5
Note 1: This ratio is generated by a 1:8 clock divider and is not an error source for current calculations.
2
_______________________________________________________________________________________
%
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
(VIN = +12V, VOUT = 6V, Circuit of Figure 2, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
PWM Maximum Duty Cycle
Input Low Voltage
Input High Voltage
Input Leakage Current
SYMBOL
CONDITIONS
MIN
TYP
MAX
100
VIL
VIH
IIN
D0, D1
D0, D1
D0, D1
UNITS
±1
%
V
V
µA
MAX
UNITS
5.5
26
V
0.8
2.4
ELECTRICAL CHARACTERISTICS
(VIN = +12V, VOUT = 6V, Circuit of Figure 2, TA = -40°C to +85°C, unless otherwise noted.)
PARAMETER
Input Voltage Range
SYMBOL
CONDITIONS
VIN
MIN
TYP
Linear-Regulator Output
Voltage, VIN Referenced
VLDOH
VIN = 5.5V to 26V,
ILOAD = 0 to 20mA
VIN 5.5
VIN 4.5
V
Linear-Regulator Output
Voltage, Ground Referenced
VLDOL
VIN = 5.5V to 26V,
ILOAD = 0 to 20mA
4.5
5.5
V
Full-Scale Current-Sense
Threshold
MAX1640
141
159
MAX1641
146
154
Quarter-Scale Current-Sense
Threshold
MAX1640
34
48
MAX1641
33
42
Output Current Compliance
VOUT = 2V to 24V (MAX1640)
Quiescent VIN Supply Current
Output Current in Off Mode
mV
mV
0.4
%/V
D0 or D1 = high
4
mA
D0 = D1 = low
1
µA
4.0
4.4
V
1.94
2.06
V
10
mV
VSET Input Current
1
µA
FET Drive Output Resistance
12
Ω
1.5
8
µs
VLDOL Undervoltage Lockout
Reference Voltage
VREF
Reference Load Regulation
IREF = 0 to 50µA
Off-Time Range
Off-Time Accuracy
RTOFF = 62kΩ
1.5
2.5
µs
Pulse-Trickle Mode Duty-Cycle
Period
D0 = low, D1 = high, RTOFF = 50kΩ
25
42
ms
PWM Maximum Duty Cycle
100
Input Low Voltage
VIL
D0, D1
Input High Voltage
VIH
D0, D1
Input Leakage Current
IIN
D0, D1
%
0.8
2.4
V
V
±1
µA
_______________________________________________________________________________________
3
MAX1640/MAX1641
ELECTRICAL CHARACTERISTICS (continued)
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
60
1.490
TA = +85°C
1.480
TA = +25°C
40
4
6
8 10 12 14 16 18 20 22 24
MAX1640/41-TOC03
TA = +85°C
1.490
1.480
1.470
TA = +25°C
1.460
1.450
1.460
2
TA = -40°C
1.500
1.470
50
4
8
12
16
20
24
2
28
4
6
8 10 12 14 16 18 20 22 24
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
MAX1641
OUTPUT CURRENT vs. INPUT VOLTAGE
MAX1641
OUTPUT CURRENT vs. OUTPUT VOLTAGE
QUIESCENT CURRENT
vs. INPUT VOLTAGE (NO-LOAD)
TA = +25°C
1.500
TA = +85°C
1.475
1.520
TA = -40°C
1.500
TA = +25°C
1.480
TA = +85°C
1.460
1.450
16
20
24
28
4
6
8 10 12 14 16 18 20 22 24
0.61
TA = +85°C
0.57
TA = +25°C
TA = -40°C
0.51
SWITCHING FREQUENCY (kHz)
0.63
10,000
1000
VOUT = +3V
100
8
12
16
24
28
LINE-TRANSIENT RESPONSE
A
0A
VOUT = +6V
10
0.49
B
0.47
0V
1
0.45
4
8
12
16
20
INPUT VOLTAGE (V)
24
28
0
50
100 150 200 250
TOFF (kΩ)
300 350 400
VLOAD = 3V
2ms/div
A: OUTPUT CURRENT, D1 = D0 = 1 1A/div
B: INPUT VOLTAGE, 10V/div
4
20
INPUT VOLTAGE (V)
SWITCHING FREQUENCY vs. RTOFF
MAX1640/41-TOC07
0.65
0.53
4
VOUT (V)
OFF-MODE SUPPLY CURRENT
(NO-LOAD)
0.55
TA = -40°C
1.9
1.5
2
INPUT VOLTAGE (V)
0.59
2.1
MAX1640/41 TOC 09
12
TA = +25°C
2.3
MAX1640/41 TOC 08
8
2.5
1.7
1.420
4
TA = +85°C
2.7
1.440
MAX1640/41-TOC06
1.525
1.540
2.9
QUIESCENT CURRENT (mA)
TA = -40°C
1.560
OUTPUT CURRENT (A)
(VOUT = 4V)
MAX1640/41 TOC04
OUTPUT VOLTAGE (V)
1.550
OUTPUT CURRENT (A)
TA = -40°C
1.500
1.510
OUTPUT CURRENT (A)
70
(VOUT = 4V)
MAX1640/41-TOC05
EFFICIENCY (%)
80
1.510
MAX1640/41 TOC02
VIN = 26V
OUTPUT CURRENT (A)
VIN = 12V
90
MAX1640/41-TOC01
100
VIN = 18V
MAX1640
OUTPUT CURRENT vs. OUTPUT VOLTAGE
MAX1640
OUTPUT CURRENT vs. INPUT VOLTAGE
EFFICIENCY vs. OUTPUT VOLTAGE
OFF-MODE SUPPLY CURRENT (mA)
MAX1640/MAX1641
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
_______________________________________________________________________________________
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
CURRENT-MODE CHANGE RESPONSE TIME
MAX1640/41 TOC 10
MAX1640/41 TOC11
EXITING OFF MODE
A
A
0A
0V
B
B
20µs/div
2ms/div
VIN = 12V, VSET = 1V, RLOAD = 4Ω, NO OUTPUT CAPACITOR
VIN = 12V, RLOAD = 4Ω
A: OUTPUT CURRENT, D0 = D1 = 0 1A/div
A: D0 = D1 = 1 2V/div
B: LOAD VOLTAGE, AC coupled, 500mV/div
B: OUTPUT CURRENT, 0.5A/div
______________________________________________________________Pin Description
PIN
NAME
FUNCTION
1
LDOL
Internal, Ground-Referenced Low-Dropout Linear Regulator Output.
Bypass with a 0.1µF capacitor in parallel with a 4.7µF capacitor to GND.
2
TOFF
Off-Time Select Input. A resistor (RTOFF) connected from this pin to GND programs the off-time for the hysteretic PWM step-down converter. This resistor also sets the period in duty-cycle mode. See Duty-Cycle
Mode and Programming the Off-Time.
3, 4
D1, D0
5
CC
Constant-Current Loop Compensation Input. Bypass with a 0.01µF capacitor to GND.
6
REF
Reference Voltage Output (VREF = 2V). Bypass with a 0.1µF capacitor to GND.
7
SET
Current Select Input. Program the desired current level by applying a voltage at SET between 0V and VREF,
(I = VSET / 13.3RSENSE). See Figure 3.
8
TERM
Maximum Output Voltage Termination Input. When VTERM exceeds the reference voltage, the comparator
resets the internal PWM latch, shutting off the external P-channel FET.
9
GND
Ground
10
CS-
Negative Current-Sense Comparator Input
11
CS+
Positive Current-Sense Comparator Input
12
PGND
High-Current Ground Return for the output drivers
13
NDRV
Gate Drive for an optional N-channel FET synchronous rectifier
14
PDRV
Gate Drive for the P-channel FET
15
LDOH
Internal, Input-Referenced Low-Dropout Linear Regulator Output.
Bypass with a 0.33µF capacitor to IN.
16
IN
Digital Inputs. Select mode of operation (Table 1).
Power-Supply Input. Input of the internal, low-dropout linear regulators.
_______________________________________________________________________________________
5
MAX1640/MAX1641
____________________________Typical Operating Characteristics (continued)
MAX1640/MAX1641
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
IN
LDOL
LDOH
REG
A1
CS+
PDRV
A2
Gm
CS-
MODE
CONTROL
SET
B
NDRV
MUX
PGND
REF
A
SEL
MAX1640
MAX1641
TERM
D0, D1
CC
TOFF
Figure 1. MAX1640/MAX1641 Functional Diagram
6
_______________________________________________________________________________________
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
4.7µF
IN
LDOH
LDOL
PDRV
0.33µF
47µF
1/2 IR7309
IN
P
LDOL
0.1µF
4.7µF
MAX1640/MAX1641
0.33µF
47µF
1/2 IR7309
LDOH
PDRV
P
0.1µF
MAX1641
D0
MAX1640
D0
D1
D1
RTOFF
RTOFF
TOFF
1/2 IR7309
NDRV
TOFF
N
1/2 IR7309
47µH
NDRV
REF
N
VOUT
PGND
REF
BATT
47µH
PGND
R3
R1
CS+
CS+
R1
SET
0.1µF
100mΩ
R4
R2
CS-
100mΩ
SET
0.1µF
R2
CSVOUT
R3
TERM
CC
BATT
GND
TERM
CC
0.01µF
GND
R4
0.01µF
Figure 2a. Standard Application Circuit
Figure 2b. Standard Application Circuit
_______________Detailed Description
part operates for 12.5% of the period set by RTOFF,
resulting in a lower current for pulse-trickle charging.
Figure 1 is the MAX1640/MAX1641 functional diagram.
Figure 2 shows the standard application circuits.
The MAX1640/MAX1641 switch-mode current sources
utilize a hysteretic, current-mode, step-down pulsewidth-modulation (PWM) topology with constant offtime. Internal comparators control the switching
mechanism. These comparators monitor the current
through a sense resistor (RSENSE) and the voltage at
TERM. When inductor current reaches the current limit
[(VCS+ - VCS-) / RSENSE], the P-channel FET turns off
and the N-channel FET synchronous rectifier turns on.
Inductor energy is delivered to the load as the current
ramps down. This ramp rate depends on RTOFF and
inductor values. When off-time expires, the P-channel
FET turns back on and the N-channel FET turns off.
Two digital inputs, D0 and D1, select between four possible current levels (Table 1). In pulse-trickle mode, the
Charge Mode: Programming the
Output Currents
The sense resistor, RSENSE, sets two charging current
levels. Choose between these two levels by holding
D0 high, and toggling D1 either high or low (Table 1).
The fast-charge current level equals V CS / R SENSE
where VCS is the full-scale current-sense voltage of
150mV. Alternatively, calculate this current by VREF /
(13.3R SENSE ). The top-off current equals V SET /
(13.3RSENSE). A resistor-divider from REF to GND programs the voltage at SET (Figure 3).
_______________________________________________________________________________________
7
MAX1640/MAX1641
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
The voltage at SET is given by:
R1 = R2 (VREF / VSET -1 ); 10kΩ < R2 < 300kΩ
where V REF = 2V and V SET is proportional to the
desired output current level.
L
MAX1641
BATT
Table 1. Selecting Output Current Levels
D1
DO
MODE
OUTPUT CURRENT (A)
0
0
OFF
0
1
Top-Off
VSET / (13.3RSENSE)
R3
CS+
RSENSE
0
1
0
Pulse-Trickle
VSET / (13.3RSENSE)
12.5% duty cycle
1
1
Fast Charge
VSET / (13.3RSENSE)
CS-
R4
TERM
Figure 4b. Setting the Maximum Output Voltage Level
The MAX1640/MAX1641 are specified for V SET
between 0V and VREF. For VSET > VREF, output current
increases linearly (with reduced accuracy) until it
clamps at VSET ≈ 4V.
MAX1640
MAX1641
REF
Pulse-Trickle Mode: Selecting the
Pulse-Trickle Current
R1
Pulling D0 low and D1 high selects pulse-trickle mode.
This current equals VSET / (13.3RSENSE) and remains
on for 12.5% of the period set by RTOFF. Pulse-trickle
current maintains full charge across the battery and
can slowly charge a cold battery before fast charging
commences.
SET
R2
PERIOD = 3.2 x 10-7 x RTOFF (sec)
Figure 3. Adjusting the Output Current Level
Off Mode: Turning Off the Output Current
Pulling D0 and D1 low turns off the P-channel FET and
hence the output current flow. This mode also controls
end of charge and protects the battery against excessive temperatures.
L
MAX1640
CS+
Setting the Maximum Output
Voltage Level
RSENSE
CSR3
BATT
TERM
R4
The maximum output voltage should be programmed to
a level higher than the output/battery voltage (ILOAD x
RLOAD). An external resistor-divider between the output
and ground (Figure 4) sets the voltage at TERM. Once
the voltage at TERM exceeds the reference, the internal
comparator turns off the P-channel FET, terminating
current flow. Select R4 in the 10kΩ to 500kΩ range.
R3 is given by:
Figure 4a. Setting the Maximum Output Voltage Level
R3 = R4 (VOUT / VTERM) -1
8
_______________________________________________________________________________________
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
generating increased ripple at the output. Select CCC
to optimize the ripple vs. loop response.
Programming the Off-Time
Synchronous Rectification
When programming the off-time, consider such factors
as maximum inductor current ripple, maximum output
voltage, inductor value, and inductor current rating. The
output current ripple is less than the inductor current ripple and depends heavily on the output capacitor’s size.
Perform the following steps to program the off-time:
1) Select the maximum output current ripple. IR(A)
2) Select the maximum output voltage. VOUT(MAX)(V)
3) Calculate the inductor value range as follows:
Synchronous rectification reduces conduction losses in
the rectifier by shunting the Schottky diode with a lowresistance MOSFET switch. In turn, efficiency increases
by about 3% to 5% at heavy loads. To prevent crossconduction or “shoot-through,” the synchronous rectifier
turns on shortly after the P-channel power MOSFET
LMIN = (VOUTMAX x 1µs) / IR
Table 2. Component Manufacturers
COMPONENT
Inductor
LMAX = (VOUTMAX x 10µs) / IR
4) Select an inductor value in this range.
5) Calculate tOFF as follows:
MOSFETs
Sense Resistor
t OFF =
L x IR
VOUTMAX
Capacitors
6) Program tOFF by selecting RTOFF from:
Rectifier
RTOFF = (29.3 x 109) x tOFF
MANUFACTURER
Sumida
CDRH125 series
Coilcraft
D03316P series
Coiltronics
UP2 series
International Rectifier
IRF7309
Siliconix
S14539DY
Dale
WSL-2010 series
IRC
LR2010-01 series
AVX
TPS series
Sprague
595D series
Motorola
Nihon
7) Calculate the switching frequency by:
MBAR5340t3
IN5817-IN5822
NSQ03A04
turns off. The synchronous rectifier remains off for 90%
of the off-time. In low-cost designs, the synchronous
rectifier FET may be replaced by a Schottky diode.
fs = 1 / (tON + tOFF)
where tON = (IR x L) / (VIN - VOUT) and IR = (VOUT x
tOFF) / L. L is the inductor value, VIN is the input voltage, VOUT is the output voltage, and IR is the output
peak-to-peak current ripple.
Note that RTOFF sets both the off-time and the pulsetrickle charge period.
Reference
The on-chip reference is laser trimmed for a precise 2V
at REF. REF can source no more than 50µA. Bypass
REF with a 0.1µF capacitor to ground.
Constant-Current Loop: AC Loop
Compensation
The constant-current loop’s output is brought out at CC.
To reduce noise due to variations in switching currents,
bypass CC with a 1nF to 100nF capacitor to ground. A
large capacitor value maintains a constant average output current but slows the loop response to changes in
switching current. A small capacitor value speeds up
the loop response to changes in switching current,
Component Selection
External Switching Transistors
The MAX1640/MAX1641 drive an enhancement-mode
P-channel MOSFET and a synchronous-rectifier Nchannel MOSFET (Table 2).
When selecting a P-channel FET, some important parameters to consider are on-resistance (rDS(ON)), maximum drain-to-source voltage (VDS max), maximum
gate-to-source voltage (V GS max), and minimum
threshold voltage (VTH min).
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I2R power losses are the greatest heat contributor for
both high-side and low-side MOSFETs. Switching losses affect the upper MOSFET only (P-channel), since the
Schottky rectifier or the N-FET body diode clamps the
switching node before the synchronous rectifier turns on.
Rectifier Diode
If an N-channel MOSFET synchronous rectifier is not
used, a Schottky rectifier is needed. The MAX1640/
_______________________________________________________________________________________
9
MAX1640/MAX1641
where V TERM = 2V and V OUT is the desired output
voltage.
MAX1640/MAX1641
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
DC IN
PDRV
I/0
D0
I/0
D1
P
MAX1640
NDRV
N
LOW-SIDE IS SHORTED
PGND
CS+
RSENSE
CH0
CSCH1
R3
T
TERM
BATT
GND
R4
Figure 5. Microcontroller Battery Charger
MAX1641’s high switching frequency demands a highspeed rectifier (Table 2). Schottky diodes such as the
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
peak current limit and that its breakdown voltage
exceeds the output voltage (VOUT). For high-temperature applications, Schottky diodes may be inadequate
due to their high leakage current; high-speed silicon
diodes such as the MUR105 or EC11FS1 can be used
instead. At heavy loads and high temperatures, the
benefits of a Schottky diode’s low forward voltage may
outweigh the disadvantage of high leakage current. If
the application uses an N-channel MOSFET synchronous rectifier, a parallel Schottky diode is usually
unnecessary except with very high charge current (> 3
amps). Best efficiency is achieved with both an
N-channel MOSFET and a Schottky diode.
Inductor Value
Refer to the section Programming the Off-Time to select
the proper inductor value. There is a trade-off between
10
inductor value, off-time, output current ripple, and
switching frequency.
__________Applications Information
All-Purpose Microcontroller Battery
Charger: NiCd, NiMH
In applications where a microcontroller is available, the
MAX1640/MAX1641 can be used as a low-cost battery
charger (Figure 5). The controller takes over fast
charge, pulse-trickle charge, charge termination, and
other smart functions. By monitoring the output voltage
at VOUT, the controller initiates fast charge (set D0 and
D1 high), terminates fast charge and initiates top-off
(set D0 high and D1 low), enters trickle charge (set D0
low and D1 high), or shuts off and terminates current
flow (set D0 and D1 low).
Layout and Grounding
Due to high current levels and fast switching waveforms, proper PC board layout is essential. High-current ground paths should be connected in a star
______________________________________________________________________________________
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
___________________Chip Information
TRANSISTOR COUNT: 1233
QSOP.EPS
________________________________________________________Package Information
______________________________________________________________________________________
11
MAX1640/MAX1641
configuration to PGND. These traces should be wide to
reduce resistance and as short as possible to reduce
stray inductance. All low-current ground paths should
be connected to GND. Place the input bypass capacitor as close as possible to the IN pin. See MAX1640 EV
kit for layout example.