APPLICATIONS INFORMATION
Stability Considerations
The constant-voltage mode feedback loop is stable without
an output capacitor provided a battery is connected to the
charger output. With no battery present, an output
capacitor is recommended to reduce ripple voltage. When
using high value, low ESR ceramic capacitors, it is
recommended to add a 1Ω resistor in series with the
capacitor. No series resistor is needed if tantalum capacitors
are used.
In constant-current mode, the PROG pin is in the feedback
loop, not the battery. The constant-current mode stability is
affected by the impedance at the PROG pin. With no
additional capacitance on the PROG pin, the charger is
stable with program resistor values as high as 20k.
However, additional capacitance on this node reduces the
maximum allowed program resistor. The pole frequency at
the PROG pin should be kept above 100kHz. Therefore, if
the PROG pin is loaded with a capacitance, C
PROG
, the
following equation can be used to calculate the maximum
resistance value for R
PROG
:
Power Dissipation
The conditions that cause the SGM4054 to reduce charge
current through thermal feedback can be approximated by
considering the power dissipated in the IC. Nearly all of
this power dissipation is generated by the internal
MOSFET—this is calculated to be approximately:
P
D
= (V
+
– V
BAT
) ·I
BAT
where P
D
is the power dissipated, V
+
is the input supply
voltage, V
BAT
is the battery voltage and IBAT is the charge
current. The approximate ambient temperature at which
the thermal feedback begins to protect the IC is:
T
A
= 120℃-P
D
θ
JA
T
A
= 120℃-(V
+
-V
BAT
) · I
BAT
·
θ
JA
Example: An SGM4054 operating from a 5V USB supply is
programmed to supply 400mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.75V.
Assuming
θ
JA
is 150℃ /W (see Board Layout
Considerations), the ambient temperature at which the
SGM4054 will begin to reduce the charge current is
approximately:
T
A
= 120
℃
- (5V - 3.75V) ·(400mA) ·150
℃
/W
T
A
= 120
℃
- 0.5W·150
℃
/W = 120
℃
- 75℃
T
A
= 45℃
The SGM4054 can be used above 45℃ ambient, but the
charge current will be reduced from 400mA. The
approximate current at a given ambient temperature can be
approximated by:
R
PROG
≤
1
2
π
⋅
10
5
⋅
C
PROG
Average, rather than instantaneous, charge current may be
of interest to the user. For example, if a switching power
supply operating in low current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
filter can be used on the PROG pin to measure the average
battery current as shown in Figure 2. A 10k resistor has
been added between the PROG pin and the filter capacitor
to ensure stability.
I
BAT
=
120
C
° −
T
A
(V
+
−
V
BAT)
⋅
θ
JA
Using the previous example with an ambient temperature
of 60℃, the charge current will be reduced to
approximately:
I
BAT
=
Figure 2. Isolating Capacitive Load on PROG Pin and Filtering
120
C
° −
60
C
°
(
5
V
−
3.75
V
)
•
150
C
°
/
W
=
60
C
°
187.5
C
°
/
A
I
BAT
= 320mA
9
SGM4054
TOREX [ TOREX SEMICONDUCTOR ]
