ISL6251EVAL2Z Intersil, ISL6251EVAL2Z Datasheet - Page 16

• Manufacturer Part Number: ISL6251EVAL2Z
• Description: EVALUATION BOARD FOR ISL6251
• Manufacturer: Intersil

Specifications of ISL6251EVAL2Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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:
Where I
less than 24mA. Substituting I
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:
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.
Q
I
C2, C4, C8 0.1 μ F/50V ceramic capacitor
C3, C7, C9 1 μ F/10V ceramic capacitor, Taiyo Yuden
rms
GATE
C1, C10
PARTS
Q1, Q2
D2, D3
C11
C5
C6
D1
D4
L
=
I
GATE
≤
BAT
I
GATE
f
s
10 μ F/25V ceramic capacitor, Taiyo Yuden
TMK325 MJ106MY X5R (3.2x2.5x1.9mm)
LMK212BJ105MG
10nF ceramic capacitor
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)
10 μ H/3.8A/26m Ω , Sumida, CDRH104R-100
30V/35m Ω , FDS6912A, Fairchild.
is the total gate drive current and should be
V
OUT
TABLE 2. COMPONENT LIST
PART NUMBERS AND MANUFACTURER
(
V
V
IN
IN
−
V
OUT
16
GATE
)
= 24mA and f
s
= 300kHz
ISL6251, ISL6251A
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
The PWM comparator gain F
is given by:
Power Stage Transfer Functions
Transfer function F
Where
Transfer function F
M
F
F
1
2
R8, R11
( )
=
PARTS
( )
S
S
R10
R12
R13
Q3
R1
R2
R3
R4
R5
R6
R7
R9
--------- .
V
11
=
=
IN
v ˆ
ω
i ˆ
d ˆ
d ˆ
L
o
esr
TABLE 2. COMPONENT LIST (Continued)
=
=
V
=
R
Signal N-channel MOSFET, 2N7002
40m Ω , ± 1%, LRC-LR2512-01-R040-F, IRC
20m Ω , ± 1%, LRC-LR2010-01-R020-F, IRC
18 Ω , ± 5%, (0805)
2.2 Ω , ± 5%, (0805)
100k Ω , ± 5%, (0805)
10k, ± 5%, (0805)
100 Ω , ± 5%, (0805)
130k, ± 1%, (0805)
10.2k Ω , ± 1%, (0805)
4.7 Ω , ± 5%, (0805)
20k Ω , ± 1%, (0805)
1.87k Ω , ± 1%, (0805)
in
o
R
V
+
c
S
ω
in
1
C
PART NUMBERS AND MANUFACTURER
R
o
2
2
o
1
2
1
L
(S) from control to output voltage is:
+
(S) from control to inductor current is:
,
+
ω
S
ω
Q
ω
o
2
2
o
p
S
S
esr
Q
1
+
≈
p
+
ω
R
+
m
ω
o
o
S
S
m
1
Q
z
for peak current mode control
p
:
C
L
o
+
1
,
, where
ω
o
=
LC
1
ω
o
z
≈
May 10, 2006
R
o
1
FN9202.2
C
o
.