BQ24618_15 Datasheet, PDF (30/43 Pages) Texas Instruments – System Power Selector and Low

bq24618
SLUSA55B – OCTOBER 2010 – REVISED APRIL 2015
www.ti.com
P_bottom = (1 - D) ´ I_CHG ² ´ R_DS(on)
(22)
If the SRP-SRN voltage decreases below 5 mV (the charger is also forced into nonsynchronous mode when the
average SRP-SRN voltage is lower than 1.25 mV), the low-side FET is turned off for the remainder of the
switching cycle to prevent negative inductor current.
As a result, all the freewheeling current goes through the body diode of the bottom-side MOSFET. The maximum
charging current in nonsynchronous mode can be up to 0.9 A (0.5 A typical) for a 10-mΩ charging current
sensing resistor, considering IC tolerance. Choose the bottom-side MOSFET with either an internal Schottky or
body diode capable of carrying the maximum nonsynchronous mode charging current.
MOSFET gate driver power loss contributes to the dominant losses on the controller IC when the buck converter
is switching. Choosing a MOSFET with a small Q_{g_total} reduces the IC power loss to avoid thermal shutdown.
P_{ICLoss_driver} = V_IN × Q_{g_total} × f_s
where
• Q_{g_total} is the total gate charge for both upper and lower MOSFET at 6 V V_REGN.
(23)
9.2.2.5 Input Filter Design
During adapter hot plug-in, the parasitic inductance and input capacitor from the adapter cable form a second-
order system. The voltage spike at the VCC pin may be beyond the IC maximum voltage rating and damage the
IC. The input filter must be carefully designed and tested to prevent an overvoltage event on the VCC pin. The
ACP/ACN pin must be placed after the input ACFET in order to avoid overvoltage stress on these pins during hot
plug-in.
There are several methods to damping or limiting the overvoltage spike during adapter hot plug-in. An electrolytic
capacitor with high ESR as an input capacitor can damp the overvoltage spike well below the IC maximum pin
voltage rating. A high current capability TVS Zener diode can also limit the overvoltage level to an IC-safe level.
However, these two solutions may not have low cost or small size.
A cost-effective and small-size solution is shown in Figure 20. R1 and C1 comprise a damping RC network to
damp the hot plug-in oscillation. As a result, the overvoltage spike is limited to a safe level. D1 is used for
reverse voltage protection for the VCC pin (it can be the body diode of the input ACFET). C2 is a VCC pin
decoupling capacitor, and it should be placed as close as possible to the VCC pin. R2 and C2 form a damping
RC network to further protect the IC from high dv/dt and high-voltage spikes. The value of C2 should be less
than the value of C1 so R1 can be dominant over the ESR orf C1 to get enough damping effect for hot plug-in.
The R1 and R2 packages must be sized to handle the inrush current power loss according to the resistor
manufacturer’s data sheet. The filter component values always must be verified with the real application, and
minor adjustments may be needed to fit in the real application circuit.
Adapter
connector
D1
R1
2W
C1
2.2 mF
(2010)
R2 ⁽¹²⁰⁶⁾
4.7 -30W
VCC pin
C2
0.1-1 mF
Figure 20. Input Filter
9.2.2.6 Inductor, Capacitor, and Sense Resistor Selection Guidelines
The bq24618 provides internal loop compensation. With this scheme, best stability occurs when the LC resonant
frequency, f_o, is approximately 12 kHz to 17 kHz for the bq24618.
The following table provides a summary of typical LC components for various charge currents:
30
Submit Documentation Feedback
Product Folder Links: bq24618
Copyright © 2010–2015, Texas Instruments Incorporated