ISL6255_14 Datasheet, PDF (18/22 Pages) Intersil Corporation – Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

ISL6255, ISL6255A
TABLE 2. COMPONENT LIST (Continued)
PARTS
PART NUMBERS AND MANUFACTURER
L
10μH/3.8A/26mΩ, Sumida, CDRH104R-100
Q1, Q2 30V/35mΩ, FDS6912A, Fairchild
Q3, Q4 -30V/30mΩ, SI4835BDY, Siliconix
Q5
Signal P-channel MOSFET, NDS352AP
Q6
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
ISL6255, ISL6255A uses a constant frequency current mode
control architecture to achieve fast loop transient response.
An accurate current sensing resistor 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
compensator to stabilize the voltage loop than voltage mode
control. Figure 17 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:
M
=
--¹---¹----
V_IN
.
Power Stage Transfer Functions
Transfer function F₁(S) from control to output voltage is:
1+ ^S
F₁ (S)
=
vˆ _o
dˆ
= V_in
S²
+
ω _esr
S
+1
ω_o2 ^ωo^Qp
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
ω
2
o
1+ ^S
ω_z
+^S
ω_oQ_p
+1
,
where
ω_z
≈¹
R_oC_o
.
Current loop gain T_i(S) is expressed as the following
equation:
T_i(S) = 0.25 R_TF₂(S)M
where R_T is the trans-resistance in current loop. R_T is
usually equal to the product of the charging current sensing
resistance and the gain of the current sense amplifier, CA2.
For ISL6255, ISL6255A, R_T = 20R₁.
The voltage gain with open current loop is:
T_v(S) = KM F₁(S)A_V(S)
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closed is given by:
L_v
(
S
)
=
T_v (S)
1 + T_i (S
)
If T_i(S)>>1, then it can be simplified as follows:
L_V(S)
=
-⁴---^V-----^F---^B--
V_O
(_---R_{-----O-----+------}R_----L----)_--
R_T
--¹-----+------^{ω---------Se}------^s-------^r
1 + _ω--^S---_P--
A_V(
S
),
ω_P
≈
--------¹---------
R_OC_O
From the above equation, it is shown that the system is a
single order system, which has a single pole located at ^ω_p
before the half switching frequency. Therefore, simple type II
compensator can be easily used to stabilize the system.
iˆ_in
iˆ_L ^L
vˆ _o
vˆ _in
+
I_Ldˆ ^{1:D Vindˆ}
R_T
Rc
Ro
V_CA2
Co
T_i(S)
dˆ
K
11/Vin
0.25V_CA2
+
-
T_v(S)
vˆ_{comp -Av(S)}
FIGURE 17. SMALL SIGNAL MODEL OF SYNCHRONOUS
BUCK REGULATOR
18
FN9203.2
May 23, 2006