SNVS085V – JULY 2000 – REVISED FEBRUARY 2013
FUNCTIONAL DESCRIPTION
The LM3478 uses a fixed frequency, Pulse Width Modulated (PWM) current mode control architecture. The block
diagram above shows the basic functionality. In a typical application circuit, the peak current through the external
MOSFET is sensed through an external sense resistor. The voltage across this resistor is fed into the I
SEN
pin.
This voltage is fed into the positive input of the PWM comparator. The output voltage is also sensed through an
external feedback resistor divider network and fed into the error amplifier negative input (feedback pin, FB). The
output of the error amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input
of the PWM comparator. At the start of any switching cycle, the oscillator sets the RS latch using the switch logic
block. This forces a high signal on the DR pin (gate of the external MOSFET) and the external MOSFET turns
on. When the voltage on the positive input of the PWM comparator exceeds the negative input, the RS latch is
reset and the external MOSFET turns off.
The voltage sensed across the sense resistor generally contains spurious noise spikes, as shown in
These spikes can force the PWM comparator to reset the RS latch prematurely. To prevent these spikes from
resetting the latch, a blank-out circuit inside the IC prevents the PWM comparator from resetting the latch for a
short duration after the latch is set. This duration is about 325ns and is called the blanking interval and is
specified as minimum on-time in the Electrical Characteristics section. Under extremely light-load or no-load
conditions, the energy delivered to the output capacitor when the external MOSFET in on during the blanking
interval is more than what is delivered to the load. An over-voltage comparator inside the LM3478 prevents the
output voltage from rising under these conditions. The over-voltage comparator senses the feedback (FB pin)
voltage and resets the RS latch. The latch remains in reset state until the output decays to the nominal value.
OVER VOLTAGE PROTECTION
The LM3478 has over voltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (pin 3).
If at anytime the voltage at the feedback pin rises to V
FB
+ V
OVP
, OVP is triggered. See ELECTRICAL
CHARACTERISTICS section for limits on V
FB
and V
OVP
.
OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage
will drop. The LM3478 will begin switching again when the feedback voltage reaches V
FB
+ (V
OVP
- V
OVP(HYS)
).
See ELECTRICAL CHARACTERISTICS for limits on V
OVP(HYS)
.
OVP can be triggered if the unregulated input voltage crosses 7.2V, the output voltage will react as shown in
The internal bias of the LM3478 comes from either the internal LDO as shown in the block diagram or
the voltage at the Vin pin is used directly. At Vin voltages lower than 7.2V the internal IC bias is the Vin voltage
and at voltages above 7.2V the internal LDO of the LM3478 provides the bias. At the switch over threshold at
7.2V a sudden small change in bias voltage is seen by all the internal blocks of the LM3478. The control voltage
shifts because of the bias change, the PWM comparator tries to keep regulation. To the PWM comparator, the
scenario is identical to a step change in the load current, so the response at the output voltage is the same as
would be observed in a step load change. Hence, the output voltage overshoot here can also trigger OVP. The
LM3478 will regulate in hysteretic mode for several cycles, or may not recover and simply stay in hysteretic
mode until the load current drops or Vin is not crossing the 7.2V threshold anymore. Note that the output is still
regulated in hysteretic mode.
Depending on the requirements of the application there is some influence one has over this effect. The threshold
of 7.2V can be shifted to higher voltages by adding a resistor in series with Vin. In case Vin is right at the
threshold of 7.2V it can happen that the threshold is crossed over and over due to some slight ripple on Vin. To
minimize the effect on the output voltage one can filter the Vin pin with an RC filter.
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