LT1103_15 Datasheet, PDF (17/32 Pages) Linear Technology – Offline Switching Regulator

LT1103/LT1105
APPLICATIONS INFORMATION
SS is reset to 0V whenever V_IN is less than 7V (prestart
mode) or when shutdown is activated by pulling V_C below
0.15V. The SS pin has a guaranteed reset sink current of
1mA when either the regulator supply voltage V_IN falls
below 7V or the regulator is placed in shutdown.
Shutdown
The LT1103/LT1105 can be put in a low quiescent current
shutdown mode by pulling V_C below 150mV. In the
shutdown mode the internal voltage regulators are turned
off, SS is reset to 0V and the part draws less than 200µA.
To initiate shutdown, about 400µA must be pulled out of
V_C until the internal voltage regulators turn off. Then, less
than 50µA pull-down current is required to maintain
shutdown. The shutdown function has about 60mV of
hysteresis on the V_C pin before the part returns to normal
operation. Soft-start, if used, controls the recovery from
shutdown.
5V Reference
A 5V reference output is available for the user’s convenience
to power primary-side circuitry or to generate a clamp
voltage for switch current limiting. The output will source
25mA and the voltage temperature coefficient is typically
50ppm/°C. If bypassing of the 5V reference is required, a
0.1µF is recommended. Values of capacitance greater
than 1µF may be susceptible to ringing due to decreased
phase margin. In such cases, the capacitive load can be
isolated from the reference output with a small series
resistor at the expense of load regulation performance.
Overvoltage Lockout
The switching supply and primarily the external power
MOSFET can be protected from an extreme surge of the
input line voltage with the overvoltage lockout feature
implemented on the OVLO pin. If the voltage on OVLO rises
above its typical threshold voltage of 2.5V, output switching
is inhibited. This feature can be implemented with a
resistive divider off of the rectified DC input voltage. This
feature is only available on the LT1105 in the 14-lead PDIP
and must be tied to ground if left unused.
Ground (LT1103)
The ground pin of the LT1103 is important because it acts
as the negative sense point for the internal error amplifier
feedback signal, the negative sense point for the current
limit amplifier, and as the high current path for the 2A
switch. The tab of the 7-lead TO-220 is internally connected
to GND (Pin 4).
To avoid degradation of load regulation, the feedback
resistor divider string and the reference side of the bias
winding should be directly connected to the ground pin on
the package. These ground connections should not be
mixed with high current carrying ground return paths. The
length of the switch current ground path should be as
short as possible to the input supply bypass capacitor and
low resistance for best performance. The case of the
LT1103 package is desirable to use as the high current
ground return path as this is a lower resistive and inductive
path than that of the actual package pin and will help
minimize voltage spikes associated with the high dI/dt
switch current.
Avoiding long wire runs to the ground pin minimizes load
regulation effects and inductive voltages created by the
high dI/dt switch current. Ground plane techniques should
also be used and will help keep EMI to a minimum.
Grounding techniques are illustrated in the Typical
Applications section.
Ground (LT1105)
The ground pin of the LT1105 is important because it acts
as the negative sense point for the internal error amplifier
feedback signal and as the negative sense point for the
current limit amplifier. The LT1105 8-pin PDIP has Pin 1
as its ground. The LT1105 14-pin PDIP has Pin 1 and
Pin 7 as grounds and must be tied together for proper
operation.
To avoid degradation of load regulation, the feedback
resistor divider should be directly connected to the package
ground pin. These ground connections should not be
mixed with high current carrying ground return paths. The
length of the switch current ground path should be as
short as possible to the input supply bypass capacitor and
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