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LTC1142HVCG Datasheet(PDF) 10 Page - Linear Technology

Part # LTC1142HVCG
Description  Dual High Efficiency Synchronous Step-Down Switching Regulators
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1142HVCG Datasheet(HTML) 10 Page - Linear Technology

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LTC1142/LTC1142L/LTC1142HV
APPLICATIO S I FOR ATIO
A graph for selecting CT versus frequency including the
effects of input voltage is given in Figure 3.
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations section). The complete expression for
operating frequency of the circuit in Figure 1 is given by:
f
t
V
V
OFF
OUT
IN
=−


1
1
where:
tC
V
V
OFF
T
REG
OUT
×
×


13 10
4
.
VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT
is the measured output voltage. Thus VREG/VOUT = 1 in
regulation.
Note that as VIN decreases, the frequency decreases.
When the input-to-output voltage differential drops below
1.5V for a particular section, the LTC1142 reduces tOFF in
that section by increasing the discharge current in CT. This
prevents audible operation prior to dropout.
Figure 2. Selecting RSENSE
MAXIMUM OUTPUT CURRENT (A)
0
0.20
0.15
0.10
0.05
0
4
1142 F02
1
2
3
5
L and CT Selection for Operating Frequency
Each regulator section of the LTC1142 uses a constant off-
time architecture with tOFF determined by an external
timing capacitor CT. Each time the P-channel MOSFET
switch turns on, the voltage on CT is reset to approximately
3.3V. During the off-time, CT is discharged by a current
which is proportional to VOUT. The voltage on CT is
analogous to the current in inductor L, which likewise
decays at a rate proportional to VOUT. Thus the inductor
value must track the timing capacitor value.
The value of CT is calculated from the desired continuous
mode operating frequency:
C
f
T =
××
1
26 10
4
.
Assumes VIN = 2VOUT, Figure 1 circuit.
Figure 3. Timing Capacitor Value
FREQUENCY (kHz)
0
50
100
150
200
1142 F03
250
1000
800
600
400
200
0
300
VIN = 12V
VIN = 10V
VIN = 7V
VSENSE = VOUT = 5V
Once the frequency has been set by CT, the inductor L must
be chosen to provide no more than 25mV/RSENSE of peak-
to-peak inductor ripple current. This results in a minimum
required inductor value of:
LMIN = 5.1 × 105 × RSENSE × CT × VREG
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor are
both track IMAX. Once RSENSE has been chosen, IBURST and
ISC(PK) can be predicted from the following:
IBURST
SENSE
SC(PK)
SENSE
15mV
R
I=
150mV
R
The LTC1142 automatically extends tOFF during a short
circuit to allow sufficient time for the inductor current to
decay between switch cycles. The resulting ripple current
causes the average short-circuit current ISC(AVG) to be
reduced to approximately IMAX.


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