Datasheet

29Maxim Integrated
1.2A Switch-Mode Li+ Chargers with ±22V Input
Rating and JEITA Battery Temperature Monitoring
MAX8900A/MAX8900B/MAX8900C
If the input voltage approaches the BAT voltage during
fast-charge constant current state, the maximum current
is no longer limited by the current loop, but by the drop-
out resistance. The total dropout resistance is:
R
DROPOUT
= R
IN2INBP
+ R
HS
+ R
L
+
R
SNS
= 0.120I + 0.100I + R
L
+ 0.040I
R
DROPOUT
heavily depends upon the inductor ESR (R
L
).
A low R
L
is required to maximize performance.
BAT Capacitor
Choose the nominal BAT capacitance to be 2.2FF. The
BAT capacitor is required to keep the BAT voltage ripple
small and to ensure regulation loop stability. The BAT
capacitor must have low impedance at the switching
frequency. Ceramic capacitors with X5R or X7R dielectric
are highly recommended due to their small size, low ESR,
and small temperature coefficients. For optimum load-
transient performance and very low output voltage ripple,
the BAT capacitor value can be increased above 2.2FF.
As the case sizes of ceramic surface-mount capaci-
tors decreases, their capacitance vs. DC bias voltage
characteristic becomes poor. Due to this characteristic,
it is possible for 0603 capacitors to perform well while
0402 capacitors of the same value perform poorly. The
MAX8900_ require a nominal BAT capacitance of 2.2FF,
however, after initial tolerance, bias voltage, aging, and
temperature derating, the capacitance must be greater
than 1.5FF. With the capacitor technology that is available
at the time the MAX8900_ was released to production, the
BAT capacitance is best achieved with a single ceramic
capacitor (X5R or X7R) in an 0603 or 0805 case size. The
capacitor voltage ratings should be 6.3V or greater.
INBP Capacitor
Choose the INBP capacitance (C
INBP
) to be 0.47FF.
Larger values of C
INBP
improve the decoupling for the
DC-DC step-down converter, but they cause a larger IN to
INBP inrush current when the input adapter is connected.
To limit the IN inrush current (I
IN
) to the 2.4A maximum
(see the Absolute Maximum Ratings section), limit C
INBP
by the maximum input voltage slew rate (V
INSR
) on IN:
C
INBP
< 2.4A/V
INSR
.
C
INBP
reduces the current peaks drawn from the battery
or input power source during switch-mode operation and
reduces switching noise in the MAX8900_. The impedance
of the input capacitor at the switching frequency should be
very low. Ceramic capacitors with X5R or X7R dielectric are
highly recommended due to their small size, low ESR, and
small temperature coefficients. For optimum noise immunity
and low input voltage ripple, the input capacitor value can
be increased. To fully utilize the Q22V input capability of the
MAX8900_, the INBP capacitor voltage rating must be 25V
or greater. Note that if V
IN
falls below V
BAT
, V
INBP
remains
at the V
BAT
potential (i.e., a -20V at IN does not pull down
INBP). Because V
INBP
never goes negative, it is possible to
use a polarized capacitor (Maxim recommends a ceramic
capacitor).
INBP is a critical discontinuous current path that requires
careful bypassing. In the PCB layout, place C
INBP
as
close as possible to the power pins (INBP and PGND)
to minimize parasitic inductance. If making connections
to the INBP capacitor through vias, ensure that the vias
are rated for the expected input current so they do not
contribute excess inductance and resistance between the
bypass capacitor and the power pins. The expected INBP
current is the same as the I
SAT
(see the Inductor Selection
section). See the PCB Layout section for more details.
The input capacitor must meet the input ripple current
requirement imposed by the step-down converter. Ceramic
capacitors are preferred due to their low ESR and resilience
to surge currents. Choose the INBP capacitor so that its tem-
perature rise due to ripple-current does not exceed approxi-
mately T
A
= +10NC. For a step-down regulator, the maxi-
mum input ripple current is half of the output current. This
maximum input ripple current occurs when the step-down
converter operates as 50% duty cycle (V
IN
= 2 x V
BAT
).
Other Capacitors
The minimum IN capacitor (C
IN
) is 0.47FF with a voltage
rating of 25V or greater. Note that although the MAX8900_
needs only 0.47FF, larger capacitors can be used. Some
specifications from USB-IF require a 2.2FF capacitor to
have proper handshaking during OTG operation. The BST
Figure 13. Calculated Fast-Charge Current vs. Dropout Voltage
CALCULATED FAST-CHARGE CURRENT
vs. DROPOUT VOLTAGE
V
IN
- V
BAT
(V)
FAST-CHARGE CURRENT (A)
0.80.60.2 0.4
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 1.0
R
L
= 300mI
I
FC
= 500mA
I
FC
= 1200mA
R
L
= 40mI