ISL6251 Datasheet, PDF (16/20 Pages) Intersil Corporation – Low Cost Multi-Chemistry Battery Charger Controller

ISL6251, ISL6251A
should be less than 80nC. Therefore, the ISL6251 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_OUT (V_IN −V_OUT )
V_IN
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 recommend 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 12.
TABLE 2. COMPONENT LIST
PARTS
PART NUMBERS AND MANUFACTURER
C1, C10 10µF/25V ceramic capacitor, Taiyo Yuden
TMK325 MJ106MY X5R (3.2x2.5x1.9mm)
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, D3 100mA/30V Schottky Diode, Central Semiconductor
D4
8A/30V Schottky rectifier, STPS8L30B (optional)
L
10µH/3.8A/26mΩ, Sumida, CDRH104R-100
Q1, Q2 30V/35mΩ, FDS6912A, Fairchild.
Q3
Signal N-channel MOSFET, 2N7002
R1
40mΩ, ±1%, LRC-LR2512-01-R040-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
100Ω, ±5%, (0805)
R8, R11 130k, ±1%, (0805)
R9
10.2kΩ, ±1%, (0805)
R10 4.7Ω, ±5%, (0805)
R12 20kΩ, ±1%, (0805)
R13 1.87kΩ, ±1%, (0805)
Loop Compensation Design
ISL6251 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 single order system. It is much easier to design a
compensator to stabilize the voltage loop than voltage mode
control. Figure 14 shows the small signal model of the
synchronous buck regulator.
PWM Comparator Gain F_m:
The PWM comparator gain Fm for peak current mode
control is given by:
F_m
=
dˆ
vˆ _comp
=¹
V_PWM
Where V_PWM is the peak-peak voltage of the PWM ramp
signal.
Current Sampling Transfer Function H_e(S):
In current loop, the current signal is sampled every switching
cycle. It has the following transfer function:
H_e (S) =
S²
ω_n2
+^S
ω_nQ_n
+1
where Q_n and
respectively.
ω_n
are
given
by
Q_n
=
−²
π
, ω_n=π
f_s,
Power Stage Transfer Functions
Transfer function F₁(S) from control to output voltage is:
F₁ (S)
=
vˆ _o
dˆ
= V_in
1+ ^S
ω _esr
S² ₊ S
+1
ω
2
o
ω_oQ_p
Where
ω _esr
=
1
R_cC_o
,
Q_p
≈ R_o
C_{o , ωo =}
L
1
LC_o
Transfer function F₂(S) from control to inductor current is:
F₂ (S) =
iˆ_L
dˆ
= ^Vin
R_o + R_L
S²
1+ ^S
ω_z
+^S
+1
,
where
ω_z
≈¹
R_oC_o
.
ω
2
o
ω_oQ_p
Current loop gain T_i(S) is expressed as the following
equation:
T_i (S ) = R_T F_mF₂ (S)H_e (S)
where R_T is the trans-resistance in current loop. R_T is
usually equal to the product of the current sensing resistance
of the current amplifier. For ISL6251, R_T=20R₁.
16
FN9202.1
June 17, 2005