SMB135 View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
Manufacturer
SMB135
Summit Microelectronics
SMB135 Datasheet PDF : 34 Pages
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SMB135
APPLICATIONS INFORMATION (CONTINUED)
EXTERNAL COMPONENTS (Figure 9)
Input and Output Capacitors
The input capacitor needs to absorb all reflected input
switching ripple current generated by the SMB135
device during charging, so that no ripple current will be
seen on the input supply. The RMS value of input ripple
current in buck type charger is given by,
Irms = Ibat
Vbat ⋅ (Vin −Vbat)
Vin
.
A 4.7uF ceramic capacitor, X5R or X7R rated, with the
0603 size and low ESR sufficiently accommodates the
above RMS current.
The output capacitor needs to ensure stability of the
charger and low output ripple voltage. A 10uF ceramic
capacitor, X5R or X7R rated, with the 0603 size and
low ESR can make operations of the SMB135 device
stable and absorb all AC portion of the inductor
switching ripple current, since the RMS value of the
output ripple current is much smaller than that of the
input ripple current.
Inductor
The inductor in a buck type charger should be selected
so that all its form-factor, cost, switching ripple and
efficiency conform to the system requirement, or
constitute the best compromise. Small dimensions,
higher inductance value usually suggests higher DCR
value. High DCR generates high conduction loss.
Lower inductance value has less DCR but creates
larger switching ripple current, which produces higher
AC loss in the magnetic core and the windings. Setting
the peak-to-peak ripple current approximately 30% of
the maximum charge current is a commonly used
method. Thus,
∆I L
=
Vinmax −Vbat
L
Vbat
⋅ Vinmax ⋅
fs
,
and,
∆I L = 30% ⋅ Ibatmax ,
where, L is inductance, fs is the switching frequency.
Diode
The rectifying diode circulates the inductor current
when the internal top FET is turned off. This causes the
forward voltage drop across the diode. Thus the diode
power loss is,
PLOSS _ DIODE
= VFD
⋅
Ibat
⋅
Vin −Vbat
Vin
.
Minimize the diode power loss by choosing a low
forward voltage diode. The reverse blocking voltage
rating that is considerably higher than the input voltage
withstands any spike voltage that might appear across
the diode. Be cautious of the reverse leakage current
that constantly bleeds a small power out the battery
cells when the battery cells aren’t charged.
BOARD LAYOUT RECOMMENDATIONS
The SMB135 only requires an inductor, a rectifying
diode, an input capacitor, an output capacitor, a sense
resistor and some bypass components, the high
side FET is internal (Figures 9, 10 and 11, Table 1).
Place an input capacitor close to the IC. Place an
inductor, a rectifying diode, and an output capacitor
close to each other. Place a VDD cap, a COMP
capacitor and a COMP resistor close to the pins. Pour
sufficiently large copper shapes on both sides of the
sense resistor, toward the output capacitor and toward
the battery cells. Pour large copper shapes on the “IN”,
“OUT” and “GND” nodes as well. If it is necessary to
route from these nodes to the other side of the board,
place enough number of vias. Accuracy of current
measurements and therefore accuracy of charge
current control are at maximum only if both the
SENSEH trace and the SENSEL trace are directly
connected to each side of the resistor pads without
contacting any shapes on their ways. Make the two
routes a differential pair if possible. Internal ground
planes and power planes quickly sink heat generated
by the SMB135, the rectifying diode, and the inductor,
furthermore reduce noise concern for the IC by
providing shielding.
Summit Microelectronics, Inc
2106 3.1 11/4/2008
16
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