LTC1436A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1436A' PDF : 28 Pages View PDF
LTC1436A
LTC1436-PLL-A/LTC1437A
APPLICATIONS INFORMATION
1:N SECONDARY WINDING
VSEC
+ R6
R5
LTC1436A
LTC1437A AUXDR
SFB
AUXFB
AUXON
ON/OFF
( ) VSEC = 1.19V
1+
R6
R5
> 13V
VOAUX
12V
+
10µF
1436 F11a
Figure 11a. 12V Output Auxiliary Regulator Using
Internal Feedback Resistors
1:N SECONDARY WINDING
VSEC
+ R6
R5
LTC1436A
LTC1437A AUXDR
SFB
AUXFB
AUXON ON/OFF
( ) VSEC = 1.19V
1+
R6
R5
( ) VOAUX = 1.19V
1+
R8
R7
VOAUX
R8
+
10µF
R7
1436 F11b
Figure 11b. 5V Output Auxiliary Regulator Using
External Feedback Resistors
VPULL-UP < 8.5V
ON/OFF
INPUT
AUXON
AUXFB –
LTC1436A
LTC1437A
AUXDR
+
1.19V REFERENCE
OUTPUT
1436 F11c
Figure 11c. Auxiliary Comparator Configuration
Minimum On-Time Considerations
Minimum on-time, tON(MIN), is the smallest amount of
time that the LTC1436A/LTC1437A are capable of turning
the top MOSFET on and off again. It is determined by
internal timing delays and the gate charge required to turn
on the top MOSFET. Low duty cycle applications may
approach this minimum on-time limit. If the duty cycle
falls below what can be accommodated by the minimum
on-time, the LTC1436A/LTC1437A will begin to skip cycles.
The output voltage will continue to be regulated, but the
ripple current and ripple voltage will increase. Therefore
this limit should be avoided.
The minimum on-time for the LTC1436A/LTC1437A in a
properly configured application is less than 300ns but
increases at low ripple current amplitudes (see Figure 12).
If an application is expected to operate close to the
minimum on-time limit, an inductor value must be chosen
that is low enough to provide sufficient ripple amplitude to
meet the minimum on-time requirement. To determine the
proper value, use the following procedure:
1. Calculate on-time at maximum supply, tON(MIN) =
(1/f)(VOUT/ VIN(MAX)).
2. Use Figure 12 to obtain the peak-to-peak inductor ripple
current as a percentage of IMAX necessary to achieve
the calculated tON(MIN).
3. Ripple amplitude ∆IL(MIN) = (% from Figure 12) (IMAX)
where IMAX = 0.1/RSENSE.
4.
LMAX
=
tON(MIN)
VIN(MAX) – VOUT
∆IL(MIN)
Choose an inductor less than or equal to the calculated
LMAX to ensure proper operation.
400
350
RECOMMENDED
300
REGION FOR MIN
ON-TIME AND
MAX EFFICIENCY
250
200
0 10 20 30 40 50 60 70
INDUCTOR RIPPLE CURRENT (% OF IMAX)
1435A F12
Figure 12. Minimum On-Time vs Inductor Ripple Current
Because of the sensitivity of the LTC1436A/LTC1437A
current comparator when operating close to the minimum
on-time limit, it is important to prevent stray magnetic flux
generated by the inductor from inducing noise on the
current sense resistor, which may occur when axial type
cores are used. By orienting the sense resistor on the
radial axis of the inductor (see Figure 13), this noise will be
minimized.
20
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