Datasheet

LM2757
www.ti.com SNVS536E –OCTOBER 2007–REVISED MAY 2013
Shutdown with Output High Impedance
The LM2757 is in shutdown mode when there is a logic Low voltage on both mode select pins (M0, M1). There
are also 450kΩ pulldown resistors connected to both mode select pins, pulling the nodes down to ground when
there is no signal present. When in shutdown, the output of the LM2757 is high impedance, allowing an external
supply to drive the output line such as in USB OTG or mobile HDMI applications. Refer to the output leakage
current graphs in the Typical Performance Characteristics section for typical leakage currents into the V
OUT
pin,
when driven by a separate supply during shutdown. Output leakage increases with temperature, with the lowest
leakage occurring at -30°C and the highest leakage at 85°C (on which the graph is based). It should be noted
when looking at the graphs, as the input voltage falls the leakage peaks at around an input voltage of 1.5V, and
then goes down as the input voltage goes to 0V. The leakage at an input voltage of 0V is the same as the
leakage current when the battery is disconnected from the circuit.
Soft Start
The LM2757 employs soft start circuitry to prevent excessive input inrush currents during startup. At startup, the
output voltage gradually rises from 0V to the nominal output voltage. This occurs in 300µs (typ.). Soft-start is
engaged when the part is enabled.
Thermal Shutdown
Protection from damage related to overheating is achieved with a thermal shutdown feature. When the junction
temperature rises to 145ºC (typ.), the part switches into shutdown mode. The LM2757 disengages thermal
shutdown when the junction temperature of the part is reduced to 135ºC (typ.). Due to the high efficiency of the
LM2757, thermal shutdown and/or thermal cycling should not be encountered when the part is operated within
specified input voltage, output current, and ambient temperature operating ratings. If thermal cycling is seen
under these conditions, the most likely cause is an inadequate PCB layout that does not allow heat to be
sufficiently dissipated out of the device.
Current Limit Protection
The LM2757 charge pump contains current limit protection circuitry that protects the device during V
OUT
fault
conditions where excessive current is drawn. Output current is limited to 250mA (typ).
Application Information
Recommended Capacitor Types
The LM2757 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors
are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance
(ESR, ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally
are not recommended for use with the LM2757 due to their high ESR, as compared to ceramic capacitors.
For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use
with the LM2757. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over
temperature (X7R: ±15% over -55ºC to 125ºC; X5R: ±15% over -55ºC to 85ºC).
Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the
LM2757. These types of capacitors typically have wide capacitance tolerance (+80%, -20%) and vary
significantly over temperature (Y5V: +22%, -82% over -30ºC to +85ºC range; Z5U: +22%, -56% over +10ºC to
+85ºC range). Under some conditions, a 1µF-rated Y5V or Z5U capacitor could have a capacitance as low as
0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum
capacitance requirements of the LM2757.
Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower
capacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using
capacitors at DC bias voltages significantly below the capacitor voltage rating will usually minimize DC bias
effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics.
Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and
capacitor manufacturers. It is strongly recommended that the LM2757 circuit be thoroughly evaluated early in the
design-in process with the mass-production capacitors of choice. This will help ensure that any such variability in
capacitance does not negatively impact circuit performance.
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