LTM8029_1208 Datasheet, PDF (13/20 Pages) Linear Technology – 36VIN, 600mA Step-Down Module Converter with 5A Quiescent Current

LTM8029
Applications Information
RUN
The LTM8029 is in shutdown when the RUN pin is low
and active when the pin is high. The rising threshold of the
RUN comparator is typically 1.15V, with a 30mV hysteresis.
This threshold is accurate when V_IN is above 4.5V. Adding
a resistor divider from V_IN to RUN programs the LTM8029
to operate only when V_IN is above a desired voltage (see
Figure 1). This rising threshold voltage, V_IN(RUN), can be
adjusted by setting the values R3 and R4 such that they
satisfy the following equation:
V_IN(RUN)
=
R3 + R4
R4
• 1.15V
where the LTM8029 should not start until V_IN is above
V_IN(RUN). Note that due to the RUN pin’s hysteresis, opera-
tion will not stop until the input falls slightly below V_IN(RUN).
LTM8029
V_IN
V_IN
R3
RUN
R4
8029 F01
Figure 1. R3 and R4 Set the Minimum
Operating Threshold Voltage
Minimum Input Voltage
The LTM8029 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. Curves detailing the minimum input voltage
of the LTM8029 for various load conditions are included
in the Typical Performance Characteristics section.
The LTM8029 features the ability to skip the off-time in
switching cycle when the input voltage approaches the
target output. This allows the LTM8029 to operate an input
voltages lower than other step-down regulators. Graphs of
minimum input voltage versus output voltage and load are
given in the Typical Performance Characteristics section.
It also means that the effective frequency during this mode
of operation will be lower than the one programmed by the
resistor connected to the RT pin, so it may be necessary
to use larger input and output capacitors, depending upon
the system requirements.
Shorted Input Protection
Care needs to be taken in systems where the output will be
held high when the input to the LTM8029 is absent. This
may occur in battery charging applications or in battery
backup systems where a battery or some other supply is
diode ORed with the LTM8029’s output. If the V_IN pin is
allowed to float and the RUN pin is held high (either by a
logic signal or because it is tied to V_IN), then the LTM8029’s
internal circuitry will pull its quiescent current through
its internal power switch. This is fine if your system can
tolerate a few milliamps in this state. If you ground the
RUN pin, the input current will drop to essentially zero.
However, if the V_IN pin is grounded while the output is
held high, then parasitic diodes inside the LTM8029 can
pull large currents from the output through the V_IN pin.
Figure 2 shows a circuit that will run only when the input
voltage is present and that protects against a shorted or
reversed input.
LTM8029
V_IN
V_IN
V_OUT
V_OUT
RUN
BIAS
RT
FB
GND
8029 F02
Figure 2. The Input Diode Prevents Shorted Input from
Discharging a Backup Battery Tied to the Output. It
Also Protects the Circuit from a Reversed Input. The
LTM8029 Runs Only When the Input is Present
8029fa
13