ISL6252 Datasheet, PDF (18/25 Pages) Intersil Corporation – Highly Integrated Battery Charger Controller for Notebook Computers

ISL6252, ISL6252A
estimated by operating at very low load current (100mA) and
measuring the ringing frequency.
CSNUB and RSNUB can be calculated from Equations 26
and 27:
C_SNUB
=
-----------------²-------------------
(2πF_ring)² ⋅ L
(EQ. 26)
R_SNUB =
-----²-----^â‹---^L------
C_SNUB
(EQ. 27)
Input Capacitor Selection
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by Equation 28:
I_RMS
=
I_BA
T
-----^V----^O----^U-----^T----^â‹---⁽---^V----^I--^N-----–-----^V----^O----^U-----^T----⁾
V_IN
(EQ. 28)
This RMS ripple current must be smaller than the rated RMS
current in the capacitor datasheet. 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, it is recommended that ceramic
capacitors or polymer capacitors from Sanyo be used due to
their small size and reasonable cost.
Table 2 shows the component lists for the typical application
circuit in Figure 2.
TABLE 2. COMPONENT LIST
PARTS
PART NUMBERS AND MANUFACTURER
C₁, C₁₀ 10µF/25V ceramic capacitor, Taiyo Yuden
TMK325 MJ106MY X5R (3.2mmx2.5mmx1.9mm)
C₂, C₄, C₈ 0.1µF/50V ceramic capacitor
C₃, C₇, C₉ 1µF/10V ceramic capacitor, Taiyo Yuden
LMK212BJ105MG
C₅
10nF ceramic capacitor
C₆
6.8nF ceramic capacitor
C₁₁
3300pF ceramic capacitor
D₁
30V/3A Schottky diode, EC31QS03L (optional)
D₂
100mA/30V Schottky Diode, Central Semiconductor
L
10µH/3.8A/26mΩ, Sumida, CDRH104R-100
Q₁, Q₂
Q₆
R₁
R₂
R₃
R₄
R₅
R₆
30V/35mΩ, FDS6912A, Fairchild
Signal N-Channel MOSFET, 2N7002
40mΩ, ±1%, LRC-LR2512-01-R040-F, IRC
20mΩ, ±1%, LRC-LR2010-01-R020-F, IRC
18Ω, ±5%, (0805)
2.2Ω, ±5%, (0805)
100kΩ, ±5%, (0805)
4.7k, ±5%, (0805)
TABLE 2. COMPONENT LIST (Continued)
PARTS
PART NUMBERS AND MANUFACTURER
R₇
R₈, R₁₁
R₉
R₁₀
R₁₂
R₁₃
100Ω, ±5%, (0805)
130k, ±1%, (0805)
10.2kΩ, ±1%, (0805)
4.7Ω, ±5%, (0805)
20kΩ, ±1%, (0805)
1.87kΩ, ±1%, (0805)
Loop Compensation Design
ISL6252 has three closed loop control modes. One controls
the output voltage when the battery is fully charged or
absent. A second controls the current into the battery when
charging and the third limits current drawn from the adapter.
The charge current and input current control loops are
compensated by a single capacitor on the ICOMP pin. The
voltage control loop is compensated by a network on the
VCOMP pin. Descriptions of these control loops and
guidelines for selecting compensation components will be
given in the following sections. Which loop controls the
output is determined by the minimum current buffer and the
minimum voltage buffer shown in Figure 1. These three
loops will be described separately.
TRANSCONDUCTANCE AMPLIFIERS GM1, GM2 AND
GM3
The ISL6252 uses several transconductance amplifiers (also
known as gm amps). Most commercially available op amps
are voltage controlled voltage sources with gain expressed
as A = V_OUT/V_IN. Transconductance amps are voltage
controlled current sources with gain expressed as
gm = I_OUT/V_IN. Transconductance gain (gm) will appear in
some of the equations for poles and zeros in the
compensation.
PWM GAIN F_M
The Pulse Width Modulator in the ISL6252 converts voltage
at VCOMP to a duty cycle by comparing VCOMP to a
triangle wave (duty = VCOMP/V_{P-P RAMP}). The low-pass
filter formed by L and C_O convert the duty cycle to a DC
output voltage (Vo = V_DCIN*duty). In ISL6252, the triangle
wave amplitude is proportional to V_DCIN. Making the ramp
amplitude proportional to DCIN makes the gain from
VCOMP to the PHASE output a constant 11 and is
independent of DCIN. For small signal AC analysis, the
battery is modeled by it’s internal resistance. The total output
resistance is the sum of the sense resistor and the internal
resistance of the MOSFETs, inductor and capacitor.
Figure 18 shows the small signal model of the pulse width
modulator (PWM), power stage, output filter and battery.
18
FN6498.3
August 25, 2010