TNY284-290
R4 feeds current into the BYPASS/MULTI-FUNCTIONAL pin,
inhibiting the internal high-voltage current source that normally
maintains the BYPASS/MULTI-FUNCTIONAL pin capacitor
voltage (C7) during the internal MOSFET off-time. This reduces
the no-load consumption of this design from 140 mW to 40 mW
at 265 VAC.
Undervoltage lockout is configured by R5 connected between
the DC bus and ENABLE/UNDERVOLTAGE pin of U1. When
present, switching is inhibited until the current in the ENABLE/
UNDERVOLTAGE pin exceeds 25
μA.
This allows the start-up
voltage to be programmed within the normal operating input
voltage range, preventing glitching of the output under abnormal
low voltage conditions and also on removal of the AC input.
In addition to the simple input pi filter (C1, L1, C2) for differential
mode EMI, this design makes use of E-Shield™ shielding
techniques in the transformer to reduce common mode EMI
displacement currents, and R2 and C4 as a damping network
to reduce high frequency transformer ringing. These techniques,
combined with the frequency jitter of TNY288, give excellent
conducted and radiated EMI performance with this design
achieving >12 dBμV of margin to EN55022 Class B conducted
EMI limits.
For design flexibility the value of C7 can be selected to pick one
of the 3 current limits options in U1. This allows the designer to
select the current limit appropriate for the application.
•
Function
BV
DSS
Line Compensated OCP
Typical OCP Change from
85 VAC to 265 VAC
UV Threshold
V
BP
Reset Voltage
Packages
TinySwitch-III
700 V
N/A
>40%
25
μA ±10%
2.6 V Typical
DIP-8C (P),
SMD-8C (G)
TinySwitch-4
725 V
Yes
<15%
25
μA ±5%
3.0 V Typical
DIP-8C (P),
eSOP-12B (K),
SO-8C (D)
Table 2.
Comparisons Between TinySwitch-III and TinySwitch-4.
TinySwitch-4 Design Considerations
Output Power Table
The data sheet output power table (Table 1) represents the
minimum practical continuous output power level that can be
obtained under the following assumed conditions:
1. The minimum DC input voltage is 100 V or higher for 85 VAC
input, or 220 V or higher for 230 VAC input or 115 VAC with
a voltage doubler. The value of the input capacitance should
be sized to meet these criteria for AC input designs.
2. Efficiency of 75%.
3. Minimum data sheet value of I
2
f.
4. Transformer primary inductance tolerance of ±10%.
5. Reflected output voltage (V
OR
) of 135 V.
6. Voltage only output of 12 V with a fast PN rectifier diode.
7. Continuous conduction mode operation with transient K
P
*
value of 0.25.
8. Increased current limit is selected for peak and open frame
power columns and standard current limit for adapter columns.
9. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper and/or a heat sink is used to keep
the SOURCE pin temperature at or below 110 °C.
10. Ambient temperature of 50 °C for open frame designs and
40 °C for sealed adapters.
*Below a value of 1, K
P
is the ratio of ripple to peak primary
current. To prevent reduced power capability due to premature
termination of switching cycles a transient K
P
limit of ≥0.25 is
recommended. This prevents the initial current limit (I
INIT
) from
being exceeded at MOSFET turn-on.
For reference, Table 3 provides the minimum practical power
delivered from each family member at the three selectable
current limit values. This assumes open frame operation (not
thermally limited) and otherwise the same conditions as listed
above. These numbers are useful to identify the correct current
limit to select for a given device and output power requirement.
Overvoltage Protection
The output overvoltage protection provided by TinySwitch-4
uses an internal latch that is triggered by a threshold current of
approximately 5.5 mA into the BYPASS/MULTI-FUNCTIONAL
pin. In addition to an internal filter, the BYPASS/MULTI-
FUNCTIONAL pin capacitor forms an external filter providing
noise immunity from inadvertent triggering. For the bypass
9
•
•
Standard current limit (I
LIMIT
) is selected with a 0.1
μF
BYPASS/
MULTI-FUNCTIONAL pin capacitor and is the normal choice
for typical enclosed adapter applications.
When a 1
μF
BYPASS/MULTI-FUNCTIONAL pin capacitor is
used, the current limit is reduced (I
LIMITred
or I
LIMIT
-1) offering
reduced RMS device currents and therefore improved
efficiency, but at the expense of maximum power capability.
This is ideal for thermally challenging designs where dissipa-
tion must be minimized.
When a 10
μF
BYPASS/MULTI-FUNCTIONAL pin capacitor is
used, the current limit is increased (I
LIMITinc
or I
LIMIT
+1), extending
the power capability for applications requiring higher peak
power or continuous power where the thermal conditions allow.
Further flexibility comes from the current limits between
adjacent TinySwitch-4 family members being compatible. The
reduced current limit of a given device is equal to the standard
current limit of the next smaller device and the increased
current limit is equal to the standard current limit of the next
larger device.
Key Application Considerations
TinySwitch-4 vs. TinySwitch-III
Table 2 compares the features and performance differences
between TinySwitch-4 and TinySwitch-III. TinySwitch-4 is pin
compatible to TinySwitch-III with improved features. It requires
minimum design effort to adapt into a new design. In addition
to the feature enhancement, TinySwitch-4 offers two new
packages; eSOP-12B (K) and SO-8C (D) to meet various
application requirements.
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Rev. A 09/12
POWERINT [ POWER INTEGRATIONS, INC. ]
