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參數(shù)資料
| 型號： | LT1738EG |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Slew Rate Controlled Ultralow Noise DC/DC Controller |
| 中文描述： | 0.3 A SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | 5.30 MM, PLASTIC, SSOP-20 |
| 文件頁數(shù)： | 17/20頁 |
| 文件大?。?/td> | 248K |
| 代理商： | LT1738EG |
17
LT1738
1738fa
The worst component from an AC point is the gate charge
current. The actual peak current depends on gate capaci-
tance and slew rate, being higher for larger values of each.
The total current can be estimated by gate charge and
frequency of operation. Because of the slewing with this
part gate charge is spread out over a longer time period
than with a normal FET driver. This reduces capacitance
requirements.
Typically the current will have spikes of under 100mA
located at the gate voltage transitions. This is charge/
discharge to and from the threshold voltage. Most slewing
occurs with the gate voltage near threshold.
Since the part’s V
IN
will typically be under 15V many
options are available for choice of capacitor. Values of
input capacitor for just the V
IN
requirement will typically be
in the 50
μ
F range with an ESR of under 0.1
.
In addition to the part’s supply, decoupling of the supply
to the inductor needs to be considered. If this is the same
supply as the V
IN
pin then that capacitor will need to be
increased. However, often with this part the inductor
supply will be a higher voltage and as such will use a
separate capacitor.
The inductor’s decoupling capacitor will see the switch
current as ripple.
The above switch current computation can be used to
estimate the capacity for these capacitors.
C
V
I
ESR
DC
f
IN
CAP
(
SW MAX
MIN
=
1
)
where
V
CAP
is the allowed sag on the input capacitor.
ESR is the equivalent series resistance for the cap. In
general allowed sag will be a few tenths of a volt.
Output Filter Capacitor
The output capacitor is chosen both for capacity and ESR.
The capacity must supply the load current in the switch on
state. While slew control reduces higher frequency com-
ponents of the ripple current in the capacitor, the capacitor
ESR and the magnitude of the output ripple current
controls the fundamental component. ESR should also be
low to reduce capacitor dissipation. Typically ESR should
be below 0.05
.
The capacitance value can be computed by consideration
of desired load ripple, duty cycle and ESR.
C
V
I
ESR
DC
f
OUT
OUT
L MAX
(
MIN
=
1
)
MOSFET Selection
There is a wide variety of MOSFETs to choose from for this
part. The part will work with either normal threshold (3V to
4V) or logic level threshold devices (1V to 2V).
Select a voltage rating to insure under worst-case condi-
tions that the MOSFET will not break down. Next choose an
R
ON
sufficiently low to meet both the power dissipation
capabilities of the MOSFET package as well as overall
efficiency needs of the converter.
The LT1738 can handle a large range of gate charges.
However at very large charge stability may be affected.
The power dissipation in the MOSFET depends on several
factors. The primary element is I
2
R heating when the
device is on. In addition, power is dissipated when the
device is slewing. An estimate for power dissipation is:
P
V
I
I
I
V
R
I
I
V
I
f I
R
DC
IN
SR
IN
ON
SR
ON
=
+
+
+
4
2
2
2
2
2
2
2
4
3
where I is the average current,
I is the ripple current in the
switch, I
SR
is the current slew rate, V
SR
is the voltage slew
rate, f is the oscillator frequency, DC is the duty cycle and
R
ON
is the MOSFET on-resistance.
Setting GCL Voltage
Setting the voltage on the GCL pin depends on what type
of MOSFET is used and the desired gate drive undervolt-
age lockout voltage.
APPLICATIU
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U
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