LT1976 Datasheet, PDF (14/24 Pages) Linear Technology – High Voltage 1.5A, 200kHz Step-Down Switching Regulator with 100mA Quiescent Current

LT1976
APPLICATIO S I FOR ATIO
I_PEAK
=
I_OUT
+
( ^V_OUT ^V_IN ^{– V}_OUT
2₍f₎₍L₎₍V_IN)
)
V_IN = maximum input voltage
f = switching frequency, 200kHz
3. Decide if the design can tolerate an “open” core geom-
etry like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid
to prevent EMI problems. This is a tough decision
because the rods or barrels are temptingly cheap and
small and there are no helpful guidelines to calculate
when the magnetic field radiation will be a problem.
4. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the Linear Technology’s applications
department if you feel uncertain about the final choice.
They have experience with a wide range of inductor
types and can tell you about the latest developments in
low profile, surface mounting, etc.
Short-Circuit Considerations
The LT1976 is a current mode controller. It uses the V_C
node voltage as an input to a current comparator which
turns off the output switch on a cycle-by-cycle basis as
this peak current is reached. The internal clamp on the V_C
node, nominally 2.2V, then acts as an output switch peak
current limit. This action becomes the switch current limit
specification. The maximum available output power is
then determined by the switch current limit.
A potential controllability prod}m could occur under short-
circuit conditions. If the power supply output is short
circuited, the feedback amplifier responds to the low
output voltage by raising the control voltage, V_C, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V_C. However, there is finite response
time involved in both the current comparator and turn-off
of the output switch. These result in a minimum on time
t_ON(MIN). When combined with the large ratio of V_IN to
(V_F + I • R), the diode forward voltage plus inductor I •␣ R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
f
•
t_ON
≤
V_F
+I•R
V_IN
where:
f = switching frequency
t_ON = switch on time
V_F = diode forward voltage
V_IN = Input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at I_PK but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT1976 clock frequency
of 200kHz, a V_IN of 40V and a (V_F + I • R) of say 0.7V, the
maximum t_ON to maintain control would be approximately
90ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscillator
to allow the current in the inductor to drop to a sufficiently
low value such that the current doesn’t continue to ratchet
higher. When the FB pin voltage is abnormally low thereby
indicating some sort of short-circuit condition, the oscil-
lator frequency will be reduced. Oscillator frequency is
reduced by a factor of 4 when the FB pin voltage is below
0.4V and increases linearly to its typical value of 200kHz at
a FB voltage of 0.95V (see Typical Performance Character-
istics). In addition, if the current in the switch exceeds 1.5
• I_PK current demanded by the V_C pin, the LT1976 will skip
the next on cycle effectively reducing the oscillator fre-
quency by a factor of 2. These oscillator frequency reduc-
tions during short-circuit conditions allow the LT1976 to
maintain current control.
SOFT-START
For applications where [V_IN/(V_OUT + V_F)] ratios > 10 or
large input surge currents can’t be tolerated, the LT1976
soft-start feature should be used to control the output
capacitor charge rate during start-up, or during recovery
from an output short circuit thereby adding additional
control over peak inductor current. The soft-start function
1976f
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