Rainbow Electronics MAX1641 - Manual (Page 7)

MAX1640/MAX1641
Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
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_______________Detailed Description
The MAX1640/MAX1641 switch-mode current sources
utilize a hysteretic, current-mode, step-down pulse-
width-modulation (PWM) topology with constant off-
time. Internal comparators control the switching
mechanism. These comparators monitor the current
through a sense resistor (R
SENSE ) and the voltage at
TERM. When inductor current reaches the current limit
[(V
CS+ - V CS- ) / R SENSE ], 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 R
TOFF 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 pos-
sible current levels (Table 1). In pulse-trickle mode, the
part operates for 12.5% of the period set by R TOFF ,
resulting in a lower current for pulse-trickle charging.
Figure 1 is the MAX1640/MAX1641 functional diagram.
Figure 2 shows the standard application circuits.
Charge Mode: Programming the
Output Currents
The sense resistor, R SENSE , 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 V
CS
is the full-scale current-sense voltage of
150mV. Alternatively, calculate this current by V REF /
(13.3R SENSE ). The top-off current equals V SET /
(13.3R
SENSE
). A resistor-divider from REF to GND pro-
grams the voltage at SET (Figure 3).
P
N
0.1 µ F
4.7 µ F
47 µ F 0.33 µ F
0.01 µ F
0.1 µ F
R1
R TOFF
R2
SET
REF
TOFF
D1
D0
LDOL PDRV
NDRV
PGND
CS+
CS-
TERM
GND CC
R3
47 µ H
V OUT
BATT
1/2 IR7309
1/2 IR7309
R4 100m Ω
LDOH IN
MAX1641
P
N
0.1 µ F
0.01 µ F
4.7 µ F
47 µ F 0.33 µ F
0.1 µ F
R1
R2
SET
REF
TOFF
D1
D0
LDOL PDRV
NDRV
PGND
CS+
CS-
R3
R4
TERM
GND CC
47 µ H
1/2 IR7309
1/2 IR7309
100m Ω
BATT
V OUT
R TOFF
LDOH IN
MAX1640
Figure 2a. Standard Application Circuit Figure 2b. Standard Application Circuit