LTC4040_15 Datasheet, PDF (20/26 Pages) Linear Technology – 2.5A Battery Backup Power Manager

LTC4040
Applications Information
Choosing the External Transistors
(MN1 and MN2) for the OVP Module and the
Input-to-Output Disconnect Switch
The LTC4040 uses a weak internal charge pump to pump
IGATE above the input voltage so that N-channel external
FETs can be used as pass transistors. However, these
transistors should be carefully chosen so that they are
fully enhanced with a VGS of 3V. Since one of these pass
transistors is the OVP FET, its breakdown voltage (BV_DSS)
determines the maximum voltage the LTC4040 can with-
stand at its input. Also, care must be taken to avoid any
leakage on the IGATE pin, as it may adversely affect the FET
operation. See Table 2 for a list of recommended transistors.
Table 2. Recommended NMOS FETs for Overvoltage Protection
and Disconnect Switch
NMOS FET
BV_DSS
R_ON
SIR424DP (Vishay)
20V
7.4mΩ
SiS488DN (Vishay)
40V
7.5mΩ
SiS424DN (Vishay)
20V
8.9mΩ
Choosing the Inductor for the Switching
Regulators
Since the same inductor is used to charge the battery in
normal mode and to deliver the system load in backup
mode, its inductance should be low enough so that the
inductor current can reverse quickly as soon as the backup
mode is initiated. On the other hand, the inductance should
not be so low that the inductor current is discontinuous
at the lowest charge current setting since charge current
accuracy suffers greatly if the inductor current is discon-
tinuous. Inductor current ripple (ΔI_L) can be computed
using the following equation:
∆I_L
=
V_BAT
•


1–
V_BAT
V_SYS


•
L
•
1
f_OSC
Since the lowest recommended charge current setting is
500mA, inductor current will be discontinuous if the ripple
is more than twice that amount, i.e, 1A. For V_SYS = 5V,
V_BAT = 3.2V, f_OSC = 2.25MHz (buck mode), and ΔI_L = 1A, the
theoretical minimum inductor size to avoid discontinuous
operation can be computed by using the above equation
to be 0.5µH. To account for inaccuracies in the system
and component values, the practical low limit should be
1µH. Since the backup boost operates at half the frequency
(1.125MHz), the inductor current ripple with a 1µH induc-
tor using the same equation will be approximately 1A in
backup mode. If this seems excessive, inductors up to
2.2µH can be used to lower the inductor current ripple.
The other considerations when choosing an inductor is
the maximum DC current (IDC) and the maximum DC
resistance (DCR) rating as shown in Table 3 below. The
chosen inductor should have a max IDC rating which is
greater than the current limit specification of the part in
order to prevent an inductor current runaway situation.
For the LTC4040, the maximum current that the inductor
can experience is approximately 8A in backup mode. It is
also important to keep the max DCR as low as possible
in order to minimize conduction loss and help improve
the converter’s efficiency.
Table 3. Recommended Inductors for the LTC4040
INDUCTOR
TYPE
MAX MAX
L IDC DCR SIZE IN mm
(µH) (A) (MΩ) (L × W × H) MANUFACTURER
XAL-5020-122 1.2 8.3 20.5 5.68 × 5.68 Coilcraft
×2
www.coilcraft.com
XAL-6030-122 1.2 10.8 7.5 6.76 × 6.76 Coilcraft
× 3.1 www.coilcraft.com
XAL-6020-132 1.3 9 15.4 6.76 × 6.76 Coilcraft
× 2.1 www.coilcraft.com
XAL-6030-182 1.8 14 10.52 6.76 × 6.76 Coilcraft
× 3.1 www.coilcraft.com
XAL-5030-222 2.2 9.2 14.5 5.3 × 5.5 Coilcraft
× 3.1 www.coilcraft.com
XAL-6030-222 2.2 15.9 13.97 6.38 × 6.58 Coilcraft
× 3.1 www.coilcraft.com
Choosing V_SYS Capacitor
The worst-case delay for the backup boost converter to
meet the system load demand can happen if the PFI input
falls below the externally set threshold at a time when the
buck charger is charging at the highest setting of 2.5A and
the system load is also very high, e.g., 2.5A. Under this
scenario, as soon as the part initiates the backup mode,
4040fa
20
For more information www.linear.com/LTC4040