LT3505 LINER [Linear Technology], LT3505 Datasheet - Page 18

• Manufacturer Part Number: LT3505
• Description: Monolithic Dual Tracking 3A Step-Down Switching
• Manufacturer: LINER [Linear Technology]

input and output voltages, and on the arrangement of the
boost circuit. The Typical Performance Characteristics
section shows plots of the minimum load current to start
and to run as a function of input voltage for 3.3V and 5V
outputs. In many cases the discharged output capacitor
will present a load to the switcher which will allow it to
start. The plots show the worst-case situation where V
ramping very slowly. Use a Schottky diode for the lowest
start-up voltage.
Frequency Compensation
The LT3501 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3501 does not require the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size.
Frequency compensation is provided by the components
tied to the V
series to ground determine loop gain. In addition, there
is a lower value capacitor in parallel. This capacitor is not
LT3501
applicaTions inForMaTion
18
V
X
= LOWEST V
OR V
V
V
BST
BST(MAX)
OUT
C
V
V
V
BST
BST(MAX)
X(MIN)
– V
pin. Generally a capacitor and a resistor in
> 3V
V
IN
IN
– V
SW
V
= V
IN
= 3V
SW
= V
= V
IN
= V
OUT
+ V
IN
X
+ V
OUT
D2
X
V
V
IN
IN
LT3501
LT3501
(5a)
(5c)
GND
GND
BST
BST
V
V
IND
OUT
IND
OUT
SW
SW
Figure 5. BST Pin Considerations
C3
C3
V
OUT
IN
< 3V
is
D2
V
OUT
part of the loop compensation but is used to filter noise
at the switching frequency.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature.
The LT1375 data sheet contains a more thorough discus-
sion of loop compensation and describes how to test the
stability using a transient load.
Figure 6 shows an equivalent circuit for the LT3501 control
loop. The error amp is a transconductance amplifier with
finite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplifier generating an output cur-
rent proportional to the voltage at the V
V
X
> V
V
V
IN
BST
BST(MAX)
V
V
V
+ 3V
BST
BST(MAX)
X(MIN)
V
V
IN
IN
– V
– V
SW
= V
= 2 • V
SW
= V
= V
IN
= V
IN
+ 3V
X
D2
D2
IN
X
V
V
IN
IN
LT3501
LT3501
(5b)
(5d)
GND
GND
BST
BST
V
V
IND
IND
OUT
OUT
SW
SW
3501 F05
C3
C
pin. Note that
V
V
OUT
OUT
< 3V
< 3V
3501fd