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參數(shù)資料
| 型號(hào)： | ISL88550AIRZ-T |
| 廠商： | INTERSIL CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Synchronous Step Down Controller with Sourcing and Sinking LDO Regulator |
| 中文描述： | SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PQCC28 |
| 封裝： | 5 X 5 MM, ROHS COMPLIANT, PLASTIC, MO-220WHHD-1, TQFN-28 |
| 文件頁數(shù)： | 18/25頁 |
| 文件大小： | 928K |
| 代理商： | ISL88550AIRZ-T |
18
FN6168.0
October 12, 2005
FIGURE 25. DUAL-MODE FEEDBACK DECODER
Setting the Buck Regulator Output (V
OUT
) with a
Resistive Voltage-Divider at FB
The Buck Regulator output voltage can be adjusted from
0.7V to 3.5V using a resistive voltage-divider (Figure 26).
The ISL88550A regulates FB to a fixed reference voltage
(0.7V). The adjusted output voltage is:
Where VFB is 0.7V and
FIGURE 26. SETTING VOUT WTH A RESISTIVE VOLTAGE
DIVIDER
Setting the VTT and VTTR Voltages (LDO)
The Termination Power Supply Output (VTT) can be set by
two different methods. First, the VTT output can be
connected directly to the VTTS input to force VTT to regulate
to V
REFIN
/2. Secondly, VTT can be forced to regulate higher
than V
REFIN
/2 by connecting a resistive divider from VTT to
VTTS. The maximum value for VTT will be the V
VTTI
-
V
DROPOUT
where V
DROPOUT
= I
VTT
× 0.3 typically.
The Termination Reference Voltage (VTTR) will follow
V
REFIN
.
Inductor Selection (Buck)
The switching frequency and inductor operating point
determine the inductor value as follows:
For example: I
LOAD(MAX)
= 12A, V
IN
= 12V, V
OUT
= 2.5V,
SW
= 300kHz, 30% ripple current or LIR = 0.3
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice, although powdered iron is
inexpensive and can work well at 200kHz. The core must be
large enough not to saturate at the peak inductor current
(I
PEAK
):
Most inductor manufacturers provide inductors in standard
values, such as 1.0μH, 1.5μH, 2.2μH, 3.3μH, etc. Also look
for nonstandard values, which can provide a better
compromise in LIR across the input voltage range. If using a
swinging inductor (where the no-load inductance decreases
linearly with increasing current), evaluate the LIR with
properly scaled inductance values.
Input Capacitor Selection (Buck)
The input capacitor must meet the ripple current requirement
(I
RMS
) imposed by the switching currents:
For most applications, non-tantalum chemistry capacitors
(ceramic, aluminum, or OSCON) are preferred due to their
resistance to power-up surge currents typical of systems
with a mechanical switch or connector in series with the
input. If the ISL88550A are operated as the second stage of
a two-stage power conversion system, tantalum input
capacitors are acceptable. In either configuration, choose a
capacitor that has less than 10°C temperature rise at the
RMS input current for optimal reliability and lifetime.
FB
0.1xREF
(0.2V)
2.5V(FIXED)
1.8V(FIXED)
OUT
REF
(2.0V)
TO ERROR
AMPLIFIER
ISL88550A
2
V
R
R
1
V
V
RIPPLE
D
C
FB
OUT
+
+
=
ESR
LOAD
I
RIPPLE
V
R
LIR
×
×
=
PHASE
L
FB
RD
RC
COUT
OUT
Q2
GND
PGND1
LGATE
ISL88550A
(
)
×
(
)
LIR
I
f
V
V
V
V
L
MAX
LOAD
SW
IN
OUT
IN
OUT
×
×
=
(
)
μH
8
=
3
×
A
12
kHz
300
V
12
V
5
×
V
12
V
5
×
L
=
(
)
+
=
2
LIR
1
I
I
MAX
LOAD
PEAK
=
IN
OUT
V
IN
OUT
V
LOAD
I
RMS
I
V
1
V
ISL88550A
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