CM9112 Datasheet, PDF (16/18 Pages) California Micro Devices Corp – Dual Inputs Dual Outputs High Accurate Fast Charger

PRELIMINARY
CM9112
Application Information (cont’d)
Adapter
Q1
10u100
1.Innormalbatteryoperation,Q2is
turnedon,thebatterysuppliespowerto
thesystemload.
2.CDMAtransmitterrequireslessthan
500mAcurrent.Inthechargingphase,the
adapter alone can supply enough power
for the CDMA system load.
GAD VIN
LDO1
VAD
0.1u
LDO2
VSYS
VREF
0.1u
System
Load
33u
Q2
Charger
CM9112
VOUT
BSEN
CDMA Phone
Figure 8. CDMA phone application
Component Selection
The constant voltage AC Adapter must be selected
carefully to minimize power losses and heat dissipation
in the charger. The input supply should be between 5.0
and 6.0V. The lowest allowable input voltage will mini­
mize heat dissipation and simplify the thermal design.
An Adapter rated at 5.0V, 5% at the required input cur­
rent will provide adequate voltage for the VAD ADOK
window.
The output of LDO1, VSYS, requires a 33µF or larger
capacitor for good stability and minimum voltage droop
during the battery switchover to VSYS at the end of
charge. A low-ESR type capacitor will improve system
response to load transients. The output of VREF
(LDO2), the Q2 gate drive, requires a .1µF ceramic
capacitor for stability.
The CM9112 drives two external P-channel MOSFETs
(PMOS) to control the charging and system currents.
Refer to Figure 9. The most important specifications for
the pass PMOS transistors are current rating, R_DS and
package power dissipation.
Adapter
5V
GAD
Q1
I_IN <= I_{SET 1}
VIN
USB
0.03
Qb
LDO1
ISYS
VSYS
4.2V
LDO2
2mA
VREF ^Q2
CM9112
GND
VOUT
VBAT ^ICH
Qc
Figure 9. Current paths
In normal operation, Q1 is a fully turned-on switch
when an AC adapter is used. The worse-case power
dissipation for the input PMOS, Q1, is:
P_Q1 = I_SET12 ⋅ R_DS
The MOSFET Q1 and PCB heatsink must be rated for
this power. Q1 functions as a clamp to limit input volt­
age transients, and should be selected to handle the
worst-case Drain-to-Source voltage, 30V is suggested.
The R_DS of Q1 should be low enough so that the volt­
age drop across it will not cause V_IN to drop below the
minimum of 4.5V when the adapter is at its lowest out­
put. For example, if the adapter is 4.75V minimum at a
load of 1.0A and ISET1 is programmed to 1.0A:
R _DS
≤^{(4.75 V 4 .5 V )}
1.0A
= 250
m∧
Q2 is used to supply power to the system from the bat­
tery when not charging (adapter removed, end-of­
charge, OCP, OTP, etc.). This current passed through
Q2. The worse case power dissipation for Q2 would
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