LT1683
20
1683fd
APPLICATIONS INFORMATION
for capacitance. However, noise depends more on the ESR
of the capacitors. In addition lower ESR can also improve
efficiency.
Input capacitors must also withstand surges that occur
during the switching of some types of loads. Some solid tan-
talum capacitors can fail under these surge conditions.
Design Note 95 offers more information but the following
is a brief summary of capacitor types and attributes.
Aluminum Electrolytic: Low cost and higher voltage. They
can be used in this application but in general you will
need higher capacitance to achieve low ESR. Additional
nonelectrolytic capacitors may be required to achieve
better performance.
Specialty Polymer Aluminum: Panasonic has come out
with their series CD capacitors. While they are only avail-
able for voltages below 16V, they have very low ESR and
good surge capability.
Solid Tantalum: Small size and low impedance. Typically
the maximum voltage rating is 50V. With large surge cur-
rents the capacitor may need to be derated or you need a
special type such as AVX TPS line.
OS-CON: Lower impedance than aluminum but only avail-
able for 35V or less. Form factor may be a problem.
Ceramic: Generally used for high frequency and high
voltage bypass. They may resonate with their ESL before
ESR becomes dominant. Recent multilayer ceramic (MLC)
capacitors provide larger capacitance with low ESR.
There are continuous improvements being made in ca-
pacitors so consult with manufacturers as to your specific
needs.
Input Capacitors
The input capacitor should have low ESR at high frequen-
cies since this will be an important factor concerning how
much conducted noise is created.
There are two separate requirements for input capacitors.
The first is for supply to the part’s V
IN
pin. The V
IN
pin
will provide current for the part itself and the gate charge
current.
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/dis-
charge 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 op-
tions are available for choice of capacitor. Values of input
capacitor for just V
IN
requirement will typically be in the
50µF range with an ESR of under 0.1Ω.
In addition to the part supply, decoupling of the supply to
the transformer 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 transformer
supply will be a higher voltage and as such a separate
capacitor.
The transformer decoupling capacitor will see the switch
current as ripple.
This switch current computation can be used to estimate
the capacity for these capacitors:
C
IN
=
∆ V
CAP
∆I
SW(MAX)
−ESR
•
MIN
f
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 volts.
Output Filter Capacitor
The output capacitor is chosen both for capacity and ESR.
The capacity must supply the load current in the switch-
off state. While slew control reduces higher frequency
components of the ripple current in the capacitor, the
capacitor ESR and the magnitude of the output ripple
