ISL6253 Datasheet, PDF (17/22 Pages) Intersil Corporation – Highly Integrated Battery Charger for Notebook Computers

ISL6253
Choose a low-side MOSFET that has the lowest possible
on-resistance, has a moderate-sized package like SO-8 and
is reasonably priced. The switching losses are not an issue
for the low side MOSFET because it operates at zero-
voltage-switching.
Choose a Schottky diode in parallel with low-side MOSFET
Q2 with a forward voltage drop low enough to prevent the
low-side MOSFET Q2 body-diode from turning on during the
dead time. This also reduces the power loss in the high-side
MOSFET associated with the reverse recovery of the low-
side MOSFET Q2 body diode.
As a general rule, select a diode with a DC current rating
equal to one-third of the load current. One option is to
choose a combined MOSFET and Schottky diode in a single
package. The integrated packages may work better in
practice because there is less stray inductance due to a
short connection. This Schottky diode is optional and may be
removed if efficiency loss can be tolerated. In addition,
ensure that the required total gate drive current for the
selected MOSFETs is less than 26mA. The total gate charge
for the high-side and low-side MOSFETs is limited by the
following equation:
Q_GATE
≤
I_{--G-----A----T---E--}
f_s
(EQ. 16)
where I_GATE is the total gate drive current and should be
less than 26mA. Substituting I_GATE = 26mA and f_s = 300kHz
into the above equation yields a total gate charge which
should be less than 86nC; therefore, the ISL6253 easily
drives the battery charge current up to 10A.
Input Capacitor Selection
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by:
I_rms = I_BAT
V_{-----O----U----T----}(_---V_{----I--N-----–----}V_{-----O----U----T----})
V_IN
(EQ. 17)
This RMS ripple current must be smaller than the rated RMS
current in the capacitor data sheet. Non-tantalum
chemistries (ceramic, aluminum, or OSCON) are preferred
due to their resistance to power-up surge currents when the
AC adapter is plugged into the battery charger. For
Notebook battery charger applications, a ceramic capacitor
or a polymer capacitor from Sanyo is recommended due to
its small size and reasonable cost.
Table 2 shows the component lists for the typical application
circuit in Figure 18.
TABLE 2. COMPONENT LIST
PARTS
PART NUMBERS AND MANUFACTURER
C1, C2
10µF/25V ceramic capacitor,
TDK, C4532X7R1E106M
C3, C5, C9, C12 0.1µF/50V ceramic capacitor
C4, C8, C10 1µF/10V ceramic capacitor,
Taiyo Yuden LMK212BJ105MG
C6
10nF ceramic capacitor
C7
6.8nF ceramic capacitor
C11
10µF or 22µF/25V/10mΩ ceramic capacitor
TDK, C5750X7R1E226M
D1
30V/3A Schottky Diode, EC31QS03L, Nihon
(optional)
D2, D3
100mA/30V Schottky Diode, Central
Semiconductor
L
15µH/4.5A/20mΩ, Sumida, CDRH127-150
Q1
30V/14mΩ, IRF7811AV, International Rectifier
Q2
30V/30mΩ, FDS6612A, Fairchild
Q3
-30V/9.5mΩ, Si4413DY, Siliconix
R1
25mΩ, ±1%, LRC-LR2010-01-R025-F, IRC
R2
20mΩ, ±1%, LRC-LR2010-01-R020-F, IRC
R3
18Ω, ±5%, (0805)
R4
2.2Ω, ±5%, (0805)
R5
100kΩ, ±5%, (0805)
R6
10k, ±5%, (0805)
R7
4.7Ω, ±5%, (0805)
R8
130kΩ, ±1%, (0805)
R9
10.2kΩ, ±1%, (0805)
R10, R11, R12 10kΩ, ±5%, (0805)
R13
100Ω, ±5%, (0805)
R14
100kΩ, ±5%, (0805)
Loop Compensation Design
ISL6253 uses constant frequency current mode control
architecture to achieve fast loop transient response.
Accurate current sensing resistors in series with the output
inductor is used to regulate the charge current, and the
sensed current signal is injected into the voltage loop to
achieve current mode control to simplify the loop
compensation design. The inductor is not considered as a
state variable for current mode control, and the system
becomes a single order system. It is much easier to design a
type II compensator to stabilize the voltage loop than voltage
mode control.
Figure 19 shows the small signal model of the synchronous
buck regulator.
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