Rev 0.95 – March 2005
The EN5310 has been optimized for use with about
20µF of ceramic output capacitance. It is strongly
recommended that these be low-cost, low-ESR,
ceramic capacitors rated X5R or X7R. (See the
Enpirion application note on ripple comparison for
optimum selection of number and value of these
capacitors based on ripple requirements.) In order to
eliminate high-frequency switching spikes on the
output ripple, usually a low-value, low-ESR ceramic
capacitor is used in parallel with the larger capacitors
right at the load.
Enable Operation
The ENABLE pin provides a means to shut down the
power FET switching or enable normal operation. A
logic low will disable the converter and cause it to
shut down. A logic high will enable the converter into
normal operation.
Soft-Start Operation
The SS pin in conjunction with a small capacitor
between this pin and AGND provides the soft start
function to limit the in-rush current during start-up.
During start-up of the converter the reference voltage
to the error amplifier is gradually increased to its final
level by an internal current source of typically 10uA.
The whole soft-start procedure is designed to take 1ms
- 3ms with a 15-30nF soft start capacitor, but can be
programmed by capacitor selection using the
following equation:
EN5310
Over-Current Protection
The cycle-by-cycle current limit function is achieved
by sensing the current flowing through the sense P-
MOSFET and a signal generated by a differential
amplifier with a preset over-current threshold. During
a particular cycle, if the over-current threshold is
exceeded, the power P-MOSFET is turned off and the
power N-MOSFET is turned on to protect the P-
MOSFET. If the over-current condition is removed,
the over-current protection circuit will enable the
PWM operation. If the over-current condition persists,
the converter will eventually go through a full soft-
start cycle. This circuit is designed to provide high
noise immunity.
It is possible to adjust the over-current set point by
connecting a resistor between ROCP and GND
(increase the trip point) or PVIN (decrease the trip
point). The voltage at the ROCP pin is designed to be
0.8V. (see application note for details)
Over-Voltage Protection
When the output voltage exceeds 120% of the
programmed output voltage, the PWM operation
stops, the lower N-MOSFET is turned on and the
POK signal goes low. When the output voltage drops
below 95% of the programmed output voltage, normal
PWM operation resumes and POK returns to its high
state.
Thermal Overload Protection
Thermal shutdown will disable operation once the
Junction temperature exceeds approximately 160ºC.
Once the junction temperature drops by approx 25ºC,
the converter will re-start with a normal soft-start.
Low Input Voltage Operation
Circuitry is provided to ensure that when the input
voltage is below the specified voltage range, the
operation of the converter is controlled and
predictable. Circuits for hysteresis, input de-glitch and
output leading edge blanking are included to ensure
high noise immunity and prevent false tripping.
Compensation
The EN5310 is internally compensated through the
use of a type 3 compensation network and is
optimized for use with about 20µF of output
capacitance and will provide excellent loop bandwidth
and transient performance for most applications. (See
Rise Time: T
R
= C
ss
*80k
POK Operation
The POK signal is an open drain signal from the
converter indicating the output voltage is within the
specified range. The POK signal will be a logic high
when the output voltage is within 90% - 120% of the
programmed output voltage. If the output voltage goes
outside of this range, the POK signal will be a logic
low until the output voltage has returned to within this
range. In the event of an over-voltage condition the
POK signal will go low and will remain in this
condition until the output voltage has dropped to 95%
of the programmed output voltage before returning to
the high state (see also Over Voltage Protection)
7
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