Max1870a step-up/step-down li+ battery charger, Battery-removal response, Table 5. mosfet resistive and switching losses – Rainbow Electronics MAX1870A User Manual
Page 28
MAX1870A
Step-Up/Step-Down
Li+ Battery Charger
28
______________________________________________________________________________________
CON) are preferred due to their resilience to power-up
surge currents.
The input capacitors should be sized so that the temper-
ature rise due to ripple current in continuous conduction
does not exceed approximately 10°C. Choose a capaci-
tor with a ripple current rating higher than 0.5 x I
CHG
.
Output-Capacitor Selection
The output capacitor absorbs the inductor ripple current
in step-down mode, or a peak-to-peak ripple current
equal to the inductor current when in step-up or hybrid
mode. As such, both capacitance and ESR are impor-
tant parameters in specifying the output capacitor. The
actual amplitude of the ripple is the combination of the
two. Ceramic devices are preferable because of their
resilience to surge currents. The worst-case output ripple
occurs during hybrid mode when the input voltage is at
its minimum. See the Typical Operating Characteristics.
Select a capacitor that can handle 0.5 x I
CHG
x V
BATT
/
V
IN
while keeping the rise in capacitor temperature less
than 10°C. Also, select the output capacitor to tolerate
the surge current delivered from the battery when it is
initially plugged into the charger.
Battery-Removal Response
Upon battery removal, the MAX1870A continues to reg-
ulate a constant inductor current until the battery volt-
age, V
BATT
, exceeds the regulation threshold. The
MAX1870A’s response time depends on the bandwidth
of the CCV loop, f
CO
(see the Voltage Loop
Compensation section). For applications where battery
overshoot is critical, either increase C
OUT
or increase
f
CO
by increasing R
CV
. See Battery Insertion and
Removal in the Typical Operating Characteristics.
System Load Transient
The MAX1870A battery charger features a very fast
response time to system load transients. Since the
input current loop is configured as a single-pole sys-
tem, the MAX1870A responds quickly to system load
transients (see the System Load-Transient Response
graph in the Typical Operating Characteristics). This
reduces the risk of tripping the overcurrent threshold of
the wall adapter and minimizes requirements for
adapter oversizing.
Table 5. MOSFET Resistive and Switching Losses
STEP-DOWN MODE
STEP-UP MODE
DESIGNATION
DC LOSSES
M1
D4
0
M2
0
D3
I
CHG
x V
DIODE
I
CHG
x V
DIODE
SWITCHING LOSSES
M1
0
D4
0
0
M2
0
D3
0
0
Note:
CLX is the total parasitic capacitance at the drain terminals of M1 and M2. IGATE is the peak gate-drive source/sink current of
M1 or M2.
V
V
x I
x R
BATT
DCIN
CHG
DS ON
2
(
)
V
V
x I
x R
BATT
DCIN
CHG
DS ON
2
(
)
1 −
V
V
x I
V
BATT
DCIN
CHG
Diode
1
2
−
V
V
x
V
V
x I
x R
DCIN
BATT
BATT
DCIN
CHG
DS ON
(
)
V
x C
x f
I
I
DCIN MAX
LX
SW CHG
GATE
(
)
2
V
x C
x f
I
I
x V
BATT MAX
LX
SW CHG
GATE
DCIN MAX
(
)
(
)
3