LTC4020_15 Datasheet, PDF (26/42 Pages) Linear Technology – 55V Buck-Boost Multi-Chemistry Battery Charger

LTC4020
APPLICATIONS INFORMATION
or large gate drive requirements. The power dissipation
in the linear pass element (P_INTVCC) is:
P_INTVCC = (V_IN – 5V) • Q_G(TOT)ABCD • f_O,
where Q_G(TOT)ABCD is the sum of all four switch total gate
charges, and f_O is the LTC4020 switching frequency.
60
50
40
30
20
V_IN
(4.5V TO 5.5V)
V_IN
INTV_CC
LTC4020
SENSVIN
4020 F10
Figure 10. Connection of Low-Voltage Input Supply
In this configuration, the INTV_CC pin cannot collapse
when the LTC4020 is in shutdown. As a result of the pin
bias being maintained during shutdown, current will flow
into the INTV_CC pin, increasing input supply current. The
total shutdown current flowing into the INTV_CC pin in this
configuration is approximately 150µA.
10
SOA
BATTERY CHARGER SECTION
0
0 10 20 30 40 50 60 70 80 90
Q_G(TOT)ABCD • f_O (mA)
4020 F08
Figure 8. INTV_CC Pass Element SOA (Safe Operating Area)
If desired operation places the internal 5V regulator out
of the allowable SOA region, deriving gate drive power
externally is required.
For driving the LTC4020 with an external 5V regulator,
connect the PV_IN and INTV_CC pins to that regulator output
as shown in Figure 9. The SENSVIN pin remains connected
to the input supply.
V_IN
V_IN
Battery Charge Voltage Programming
The LTC4020 uses an external feedback resistive divider
from the BAT pin to ground to program battery voltages.
This divider provides feedback to the VFB pin, and sets
the final voltage that the battery charger will achieve at
the end of a charge cycle. The feedback reference of 2.5V
corresponds to the battery float voltage during CC/CV
mode charging (MODE = 0V).
VBAT
LTC4020
VFB
(BATTERY)
R_FB1
R_FB2
5V_OUT
4020 F11
Figure 11. Battery Voltage Programming
V_IN
INTV_CC
LTC4020
SENSVIN
(5V)
4020 F09
The resultant feedback signal is compared with the internal
2.5V voltage reference by the converter error amplifier.
The output voltage is given by the equation:
Figure 9. Connection of External 5V Regulator for Reduced
Internal Power Dissipation
For operation with tightly regulated low voltage input sup-
plies (4.5V to 5.5V), the LTC4020 internal gate drivers and
BST refresh functions can be powered directly by the input
supply, eliminating the requirement for a 5V regulator to
supply the INTV_CC pin. Connect the input supply to the
PV_IN, INTV_CC, and SENSVIN pins, as shown in Figure 10.
V_{(FLOAT(CC/CV)}
=
2.5V
₁₊
R_FB1
R_FB2


where R_FB1 and R_FB2 are defined as in Figure 11.
If charging in CC mode (MODE = -NC-), R_FB1 and R_FB2
corresponding to V_FB = 2.5V programs a maximum V_BAT
voltage, if constant-voltage functionality at that level if
desired.
4020fb
26
For more information www.linear.com/LTC4020