APPLICATIONS INFORMATION
V+ Bypass Capacitor
Many types of capacitors can be used for input bypassing,
however, caution must be exercised when using multilayer
ceramic capacitors. Because of the self-resonant and high Q
characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions, such as connecting the charger input to a live
power source. Adding a 1.5W resistor in series with an X5R
ceramic capacitor will minimize start-up voltage transients.
For more information, refer to Application Note 88.
Figure 3．A Circuit to Maximize Thermal Mode Charge Current
2
Charge Current Soft-Start
Solving for IBAT using the quadratic formula .
The SGM4054 includes a soft-start circuit to minimize the
inrush current at the start of a charge cycle. When a charge
cycle is initiated, the charge current ramps from zero to the
full-scale current over a period of approximately 100µs.
This has the effect of minimizing the transient current load
on the power supply during start-up.
I
BAT
=
4RCC (120C° − T )
A
2
(V
S
−VBAT ) − (V
S
−VBAT
)
120C°
θ
JA
2RCC
Using RCC = 0.25W, V
JA = 125°C/W we can calculate the thermally regulated
charge current to be:
S
= 5V, VBAT = 3.75V, T
A
= 25°C and
θ
CHGR Status Output Pin
I
BAT = 708.4mA
The
pin can provide an indication that the input
CHGR
While this application delivers more energy to the battery
and reduces charge time in thermal mode, it may actually
lengthen charge time in voltage mode if V becomes low
enough to put the SGM4054 into dropout. Figure 4 shows
voltage is greater than the undervoltage lockout threshold
level. A weak pull-down current of approximately 20µA
+
indicates that sufficient voltage is applied to V to begin
+
charging. When a discharged battery is connected to the
charger, the constant current portion of the charge cycle
how this circuit can result in dropout as RCC becomes large.
begins and the
pin can sink up to 10mA to drive an LED that indicates that
a charge cycle is in progress.
pin pulls to ground. The
CHGR
CHGR
This technique works best when RCC values are minimized
to keep component size small and avoid dropout.
Remember to choose a resistor with adequate power
handling capability.
When the battery is nearing full charge, the charger enters
the constant-voltage portion of the charge cycle and the
charge current begins to drop. When the charge current
drops below 1/10 of the programmed current, the charge
cycle ends and the strong pull-down is replaced by the
20µA pull-down, indicating that the charge cycle has
ended. If the input voltage is removed or drops below the
1000
VS = 5V
CONSTANT
CURRENT
800
600
DROPOUT
VS = 5.5V
VS = 5.25V
undervoltage lockout threshold, the
pin becomes
CHGR
400
200
0
THERMAL
MODE
high impedance. Figure 5 shows that by using two
different value pull-up resistors, a microprocessor can
detect all three states from this pin.
VBAT = 3.75V
T
A = 25
JA = 125
RPROG = 1.25kΩ
0
0.25
0.5
0.75
1.0
1.25
1.5
1.75
RCC (Ω)
Figure 4. Charge Current vs. RCC
11
SGM4054
TOREX [ Torex Semiconductor ]
