LTC1142L_15 Datasheet, PDF (10/20 Pages) Linear Technology – Dual High Efficiency Synchronous Step-Down Switching Regulators

LTC1142/LTC1142L/LTC1142HV
APPLICATIO S I FOR ATIO
both track I_MAX. Once R_SENSE has been chosen, I_BURST and
I_SC(PK) can be predicted from the following:
I_BURST
≈
15mV
R_SENSE
I_SC(PK)
=
150mV
R_SENSE
The LTC1142 automatically extends t_OFF 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 I_SC(AVG) to be
reduced to approximately I_MAX.
0.20
0.15
0.10
0.05
0
0
1
2
3
4
5
MAXIMUM OUTPUT CURRENT (A)
1142 F02
Figure 2. Selecting R_SENSE
L and C_T Selection for Operating Frequency
Each regulator section of the LTC1142 uses a constant off-
time architecture with t_OFF determined by an external
timing capacitor C_T. Each time the P-channel MOSFET
switch turns on, the voltage on C_T is reset to approximately
3.3V. During the off-time, C_T is discharged by a current
which is proportional to V_OUT. The voltage on C_T is
analogous to the current in inductor L, which likewise
decays at a rate proportional to V_OUT. Thus the inductor
value must track the timing capacitor value.
The value of C_T is calculated from the desired continuous
mode operating frequency:
C_T
=
2.6
1
• 10⁴
•
f
Assumes V_IN = 2V_OUT, Figure 1 circuit.
A graph for selecting C_T 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
=
1
t_OFF

^1−
V_OUT ^
V_IN 
where:
t_OFF
=
1.3 • 10⁴
• C_T
•


V_REG
V_OUT


V_REG is the desired output voltage (i.e., 5V, 3.3V). V_OUT
is the measured output voltage. Thus V_REG/ V_OUT = 1 in
regulation.
Note that as V_IN decreases, the frequency decreases.
When the input-to-output voltage differential drops below
1.5V for a particular section, the LTC1142 reduces t_OFF in
that section by increasing the discharge current in C_T. This
prevents audible operation prior to dropout.
1000
V_SENSE = V_OUT = 5V
800
600
V_IN = 12V
400
V_IN = 7V
200
V_IN = 10V
0
0 50 100 150 200 250 300
FREQUENCY (kHz)
1142 F03
Figure 3. Timing Capacitor Value
Once the frequency has been set by C_T, the inductor L must
be chosen to provide no more than 25mV/R_SENSE of peak-
to-peak inductor ripple current. This results in a minimum
required inductor value of:
L_MIN = 5.1 • 10⁵ • R_SENSE • C_T • V_REG
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor are
10