LTC1142-ADJ Linear Technology, LTC1142-ADJ Datasheet - Page 10

• Manufacturer Part Number: LTC1142-ADJ
• Description: Dual High Efficiency Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1142/LTC1142L/LTC1142HV
both track I
I
The LTC1142 automatically extends t
circuit to allow sufficient time for the inductor current to
decay between switch cycles. The resulting ripple current
causes the average short-circuit current I
reduced to approximately I
L and C
Each regulator section of the LTC1142 uses a constant off-
time architecture with t
timing capacitor C
switch turns on, the voltage on C
3.3V. During the off-time, C
which is proportional to V
analogous to the current in inductor L, which likewise
decays at a rate proportional to V
value must track the timing capacitor value.
The value of C
mode operating frequency:
Assumes V
10
SC(PK)
I
I
C
BURST
SC(PK)
T
can be predicted from the following:
T
2 6 10
0.20
0.15
0.10
0.05
Selection for Operating Frequency
=
MAX
0
IN
150mV
R
R
15mV
0
SENSE
SENSE
= 2V
T
. Once R
1
is calculated from the desired continuous
Figure 2. Selecting R
MAXIMUM OUTPUT CURRENT (A)
4
1
OUT
T
U U
. Each time the P-channel MOSFET
f
, Figure 1 circuit.
SENSE
2
OFF
MAX
T
OUT
has been chosen, I
determined by an external
is discharged by a current
3
.
T
. The voltage on C
is reset to approximately
OUT
SENSE
W
4
. Thus the inductor
OFF
1142 F02
during a short
5
SC(AVG)
BURST
U
to be
and
T
is
A graph for selecting C
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:
where:
V
is the measured output voltage. Thus V
regulation.
Note that as V
When the input-to-output voltage differential drops below
1.5V for a particular section, the LTC1142 reduces t
that section by increasing the discharge current in C
prevents audible operation prior to dropout.
Once the frequency has been set by C
be chosen to provide no more than 25mV/R
to-peak inductor ripple current. This results in a minimum
required inductor value of:
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor are
REG
f
t
L
OFF
MIN
is the desired output voltage (i.e., 5V, 3.3V). V
t
OFF
1
= 5.1 10
1000
1 3 10
800
600
400
200
0
1
0
Figure 3. Timing Capacitor Value
IN
V
V
IN
V
OUT
50
4
IN
= 7V
decreases, the frequency decreases.
5
C
100
FREQUENCY (kHz)
R
T
T
SENSE
versus frequency including the
V
IN
150
= 12V
V
V
V
OUT
REG
SENSE
200
C
T
= V
T
OUT
V
, the inductor L must
IN
250
V
= 10V
= 5V
REG
1142 F03
REG
300
SENSE
/ V
OUT
of peak-
T
= 1 in
OFF
. This
OUT
in