ISL6251_06 Datasheet, PDF (16/20 Pages) Intersil Corporation – Low Cost Multi-Chemistry Battery Charger Controller

ISL6251, ISL6251A
As a general rule, select a diode with DC current rating equal
to one-third of the load current. One option is to choose a
combined MOSFET with the 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 should be less than 24mA. So, the total
gate charge for the high-side and low-side MOSFETs is
limited by the following equation:
Q_GATE
≤
I_GATE
f_s
Where I_GATE is the total gate drive current and should be
less than 24mA. Substituting I_GATE = 24mA and f_s = 300kHz
into the above equation yields that the total gate charge
should be less than 80nC. Therefore, the ISL6251,
ISL6251A 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
C11
D1
D2, D3
D4
L
Q1, Q2
6.8nF ceramic capacitor
3300pF ceramic capacitor
30V/3A Schottky diode, EC31QS03L (optional)
100mA/30V Schottky Diode, Central Semiconductor
8A/30V Schottky rectifier, STPS8L30B (optional)
10μH/3.8A/26mΩ, Sumida, CDRH104R-100
30V/35mΩ, FDS6912A, Fairchild.
TABLE 2. COMPONENT LIST (Continued)
PARTS
PART NUMBERS AND MANUFACTURER
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, ISL6251A uses 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 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 F_m 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:
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
16
FN9202.2
May 10, 2006