1-27
IC
VGN-
S36C/S36GP/S36LP/S36SP/S36TP/S38CP/
S52B/
S62PS/S62PSY/S62S/
S350F/S350FP/
S360/S360P/
S370F
(J/AM
/AO
)
Confidential
Setting the Battery Regulation Voltage
The MAX1908 uses a high-accuracy voltage regulator
for charging voltage. The VCTL input adjusts the
charger output voltage. VCTL control voltage can vary
from 0 to V
REFIN
, providing a 10% adjustment range on
V
BATT
regulation voltage. By limiting the adjust range to
10% of the regulation voltage, the external resistor
mismatch error is reduced from 2% to 0.2% of the
regulation voltage. Therefore, an overall voltage accu-
racy of better than 0.7% is maintained while using 1%
resistors. The per-cell battery termination voltage is a
function of the battery chemistry. Consult the battery
manufacturer to determine this voltage. Connect VCTL
to LDO to select the internal default setting V
BATT
=
4.2V
×
number of cells, or program the battery voltage
with the following equation:
CELLS is the programming input for selecting cell
count. Connect CELLS as shown in Table 1 to charge
2, 3, or 4 Li+ cells. When charging other cell
chemistries, use CELLS to select an output voltage
range for the charger.
The internal error amplifier (GMV) maintains voltage
regulation (Figure 3). The voltage error amplifier is
compensated at CCV. The component values shown in
Figures 1 and 2 provide suitable performance for most
applications. Individual compensation of the voltage
regulation and current-regulation loops allows for opti-
mal compensation (see the
Compensation
section).
V
CELLS
V
V
V
BATT
VCTL
REFIN
=
×
+
×
4
0 4
.
Table 1. Cell-Count Programming
CELLS
CELL COUNT
GND
2
Float
3
V
REFIN
4
Pin Description
PIN
NAME
FUNCTION
1
DCIN
Charging Voltage Input. Bypass DCIN with a 1µF capacitor to PGND.
2
LDO
D evi ce P ow er S up p l y. Outp ut of the 5.4V l i near r eg ul ator sup p l i ed fr om D C IN . Byp ass w i th a 1µF cap aci tor to GN D .
3
CLS
Source Current-Limit Input. Voltage input for setting the current limit of the input source.
4
REF
4.096V Voltage Reference. Bypass REF with a 1µF capacitor to GND.
5
CCS
Input Current Regulation Loop Compensation Point. Connect a 0.01µF capacitor to GND.
6
CCI
Output Current Regulation Loop Compensation Point. Connect a 0.01µF capacitor to GND.
7
CCV
Voltage Regulation Loop Compensation Point. Connect 1k
Ω
in series with 0.1µF capacitor to GND.
8
SHDN
Shutdown Control Input. Drive
SHDN
logic low to shut down the MAX1908. Use with a thermistor to detect a
hot battery and suspend charging.
9
ICHG
Charge Current Monitor Output. ICHG is a scaled-down replica of the charger output current. Use ICHG to
monitor the charging current and detect when the chip changes from constant-current mode to constant-
voltage mode. The transconductance of (CSIP - CSIN) to ICHG is 3µA/mV.
10
ACIN
AC Detect Input. Input to an uncommitted comparator. ACIN can be used to detect AC adapter presence.
11
ACOK
AC Detect Output. High-voltage open-drain output is high impedance when VACIN is less than VREF/2.
12
REFIN
Reference Input. Allows the ICTL and VCTL inputs to have ratiometric ranges for increased accuracy.
13
ICTL
Output Current-Limit Set Input. ICTL input voltage range is VREFIN/32 to VREFIN. The device shuts down if
ICTL is forced below VREFIN/100. When ICTL is equal to LDO, the set point for CSIP - CSIN is 45mV.
14
GND
Analog Ground
15
VCTL
Output-Voltage Limit Set Input. VCTL input voltage range is 0 to VREFIN. When VCTL is equal to LDO, the set
point is (4.2 x CELLS) V.
16
BATT
Battery Voltage Input
17
CELLS
Cell Count Input. Trilevel input for setting number of cells. GND = 2 cells, float = 3 cells, REFIN = 4 cells.
18
CSIN
Output Current-Sense Negative Input
19
CSIP
Output Current-Sense Positive Input. Connect a current-sense resistor from CSIP to CSIN.
20
PGND
Power Ground
21
DLO
Low-Side Power MOSFET Driver Output. Connect to low-side NMOS gate.
22
DLOV
Low-Side Driver Supply. Bypass DLOV with 1µF capacitor to GND.
23
LX
High-Side Power MOSFET Driver Power-Return Connection. Connect to source of high-side NMOS.
24
BST
High-Side Power MOSFET Driver Power-Supply Connection. Connect a 0.1µF capacitor from LX to BST.
25
DHI
High-Side Power MOSFET Driver Output. Connect to high-side NMOS gate.
26
CSSN
Input Current-Sense Negative Input
27
CSSP
Input Current-Sense Positive Input. Connect a current-sense resistor from CSSP to CSSN.
28
IINP
Input Current Monitor Output. IINP is a scaled-down replica of the input current. IINP monitors the total
system current. The transconductance of (CSSP - CSSN) to IINP is 3µA/mV.
Detailed Description
The MAX1908 includes all the functions necessary to
charge Li+ batteries. A high-efficiency synchronous-
rectified step-down DC-to-DC converter controls charg-
ing voltage and current. The device also includes input
source current limiting and analog inputs for setting the
charge current and charge voltage. Control charge
current and voltage using the ICTL and VCTL inputs,
respectively. Both ICTL and VCTL are ratiometric with
respect to REFIN, allowing compatibility with D/As or
microcontrollers (µCs). Ratiometric ICTL and VCTL
improves the accuracy of the charge current and
voltage set point by matching V
REFIN
to the reference
of the host. For standard applications, internal set
points for ICTL and VCTL provide 3A charge current
(with 0.015
Ω
sense resistor), and 4.2V (per cell) charge
voltage. Connect ICTL and VCTL to LDO to select the
internal set points. The MAX1908 safely conditions
overdischarged cells with 300mA (with 0.015
Ω
sense
resistor) until the battery pack voltage exceeds 3.1V
×
number of series-connected cells. The
SHDN
input
allows shutdown from a microcontroller or thermistor.
The DC-to-DC converter uses external N-channel
MOSFETs as the buck switch and synchronous rectifier
to convert the input voltage to the required charging
current and voltage. The typical application circuit
shown in Figure 1 uses a µC to control charging cur-
rent, while Figure 2 shows a typical application with
charging voltage and current fixed to specific values
for the application. The voltage at ICTL and the value of
RS2 set the charging current. The DC-DC converter
generates the control signals for the external MOSFETs
to regulate the voltage and the current set by the VCTL,
ICTL, and CELLS inputs.
The MAX1908 features a voltage-regulation loop (CCV)
and two current-regulation loops (CCI and CCS). The
CCV voltage-regulation loop monitors BATT to ensure
that its voltage does not exceed the voltage set by
VCTL. The CCI battery current-regulation loop monitors
current delivered to BATT to ensure that it does not
exceed the current limit set by ICTL. A third loop (CCS)
takes control and reduces the battery-charging current
when the sum of the system load and the battery-
charging input current exceeds the input current limit
set by CLS.
MAX1908
LOGIC
BLOCK
GMS
SHDN
GND
CLS
CCS
CSSP
CSSN
CSIP
CSIN
ICTL
CCI
BATT
CELLS
CCV
VCTL
23.5%
REFIN
GND
DCIN
SRDY
5.4V
LINEAR
REGULATOR
1/55
ICTL
REF/2
RDY
4V
CELL
SELECT
LOGIC
4.096V
REFERENCE
LVC
REFIN
CSI
300mA
3.1V/CELL
R1
LVC
DCIN
LDO
REF
REFIN
ACIN
ACOK
IINP
ICHG
BST
DHI
LX
DLOV
DLO
PGND
BATTERY UNDERVOLTAGE
X
75mV
REF
LEVEL
SHIFTER
X
75mV
REFIN
X
400mV
REFIN
DC-TO-DC
CONVERTER
GMI
GMV
GM
LEVEL
SHIFTER
N
GM
LEVEL
SHIFTER
DRIVER
DRIVER
BLOCK DIAGRAM
