LM3743_08 Datasheet, PDF (17/28 Pages) National Semiconductor (TI) – High-Performance Synchronous Buck Controller with Comprehensive Fault Protection Features

protection for short circuits from switch node to ground or the
case when the inductor is shorted, which the low side current
limit cannot detect. A 200 ns blanking time period after the
high-side FET turns on is used to prevent switching transient
voltages from tripping the high-side current limit without
cause. If the difference between V_CC pin and SW pin voltage
exceeds 500 mV, the LM3743 will immediately enter hiccup
mode (see Hiccup Mode section).
OUTPUT UNDER-VOLTAGE PROTECTION (UVP)
After the end of soft-start the output UVP comparator is acti-
vated. The threshold is 50% of the feedback voltage. Once
the comparator indicates UVP for more than 7 µs typ. (glitch
filter time), the IC goes into hiccup mode.
HICCUP MODE AND INTERNAL SOFT-START
Hiccup protection mode is designed to protect the external
components of the circuit (output inductor, FETs, and input
voltage source) from thermal stress. During hiccup mode, the
LM3743 disables both the high-side and low-side FETs and
begins a cool down period of 5.5 ms. At the conclusion of this
cool down period, the regulator performs an internal 3.6 ms
soft-start. There are three distinct conditions under which the
IC will enter the hiccup protection mode:
1. The low-side current sensing threshold has exceeded
the current limit threshold for fifteen sampled cycles, see
Figure 6. Each cycle is sampled at the start of each off
time (t_OFF). The low-side current limit counter is reset
when 32 consecutive non-current limit cycles occur in
between two current limit events.
2. The high-side current limit comparator has sensed a
differential voltage larger than 500 mV.
3. The voltage at the FB pin has fallen below 0.4V, and the
UVP comparator has sensed this condition for 7 μs
(during steady state operation).
The band gap reference, the external soft-start, and internal
hiccup soft-start of 3.6 ms (typ) connect to the non-inverting
input of the error amplifier through a multiplexer. The lowest
voltage of the three connects directly to the non-inverting in-
put. Hiccup mode will not discharge the external soft-start,
only UVLO or shut-down will. When in hiccup mode the inter-
nal 5.5 ms timer is set, and the internal soft-start capacitor is
discharged. After the 5.5 ms timeout, the internal 3.6 ms soft-
start begins, see Figure 7. During soft-start, only low-side
current limit and high side current limit can put the LM3743
into hiccup mode.
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FIGURE 6. Entering Hiccup Mode
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FIGURE 7. Hiccup Time-Out and Internal Soft-Start
For example, if the low-side current limit is 10A, then once in
overload the low-side current limit controls the valley current
and only allows an average amount of 10A plus the ripple
current to pass through the inductor and FETs for 15 switching
cycles. In such an amount of time, the temperature rise is very
small. Once in hiccup mode, the average current through the
high-side FET is:
I_HSF-AVE = (I_CLIM + ΔI) x [ D(15 cycles x T_SW) ] / 5.5 ms
equals 71 mA. With an arbitrary D = 60%, ripple current of 3A,
and a 300 kHz switching frequency.
The average current through the low-side FET is:
I_LSF-AVE = (I_CLIM + ΔI) x [ (1–D) x (15 cycles x T_SW) ] / 5.5 ms
equals 47 mA,
And the average current through the inductor is:
I_L-AVE = (I_CLIM + ΔI) x [ (15 cycles x T_SW) ] / 5.5 ms
equals 118 mA.
DESIGN CONSIDERATIONS
The following is a design procedure for selecting all the com-
ponents in the Typical Application circuit on the front page.
This design converts 5V (V_IN) to 1.8V (V_OUT) at a maximum
load of 10A with an efficiency of 90% and a switching fre-
quency of 300 kHz. The same procedures can be followed to
create many other designs with varying input voltages, output
voltages, load currents, and switching frequency.
Switching Frequency
Selection of the operating switching frequency is based on
trade-offs between size, cost, efficiency, and response time.
For example, a lower frequency will require larger more ex-
pensive inductors and capacitors. While a higher switching
frequency will generally reduce the size of these components,
but will have a reduction in efficiency. Fast switching convert-
ers allow for higher loop gain bandwidths, which in turn have
the ability to respond quickly to load and line transients. For
the example application we have chosen a 300 kHz switching
frequency because it will reduce the switching power losses
and in turn allow for higher conduction losses considering the
same power loss criteria, thus it is possible to sustain a higher
load current.
Output Inductor
The output inductor is responsible for smoothing the square
wave created by the switching action and for controlling the
output current ripple (ΔI_OUT) also called the AC component of
the inductor current. The DC current into the load is equal to
the average current flowing in the inductor. The inductance is
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