LT1501 LINER [Linear Technology], LT1501 Datasheet - Page 11

• Manufacturer Part Number: LT1501
• Description: Adaptive-Frequency Current Mode Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
type TPS from AVX, or type 595D from Sprague. These
have an ESR of 0.06 in a “E” case size. At lower output
current levels, a 100 F unit in a “D” case size may be
sufficient. Output ripple voltage can be calculated from:
Loop frequency stability is affected by the characteristics
of the output capacitor. The ESR of the capacitor should be
very low, and the capacitance must be large (> 200 F) to
ensure good loop stability under worst-case conditions of
low input voltage, higher output voltages, and high load
currents. The 14-pin LT1500 can use external frequency
compensation on the V
smaller output capacitors. See Loop Stability section for
details.
Precautions regarding solid tantalum capacitors for input
bypassing do not apply to the output capacitor because
turn-on surges are limited by the inductor and discharge
surges do not harm the capacitors.
Setting Output Voltage
Preset 3.3V and 5V parts are available. For other voltage
applications the adjustable part uses an external resistor
divider to set output voltage. Bias current for the feedback
(FB) pin is typically 30nA (it is internally compensated).
Thevenin divider resistance should be 100k or less to
keep bias current errors to a minimum. This leads to a
value for R1 and R2 (see Figure 1) of:
Example: V
R
R
R
V
RIPPLE
1
2
1
100
100
V
OUT
R
1 265
OUT
1 265
1 1 265
k
k
.
.
ESR
– .
.
= xxV
12
V
1 265
OUT
V
U
0 1
.
C
949
pin to give good loop stability with
INFORMATION
1 2
U
k
.
(use 1M
I
OUT
V
IN
W
V
OUT
)
U
Note that there is an internal switch that disconnects the
internal divider for fixed 3.3V and 5V parts in shutdown.
This prevents the divider from adding to shutdown cur-
rent. Without this switch, shutdown current increases
because of the divider current directly, but even more so
if the FB pin is held above 0.6V by the divider. See graphs
in Typical Performance Characteristics.
Note that there is a switch in series with the V
switch is turned off in shutdown to eliminate shutdown
current drawn by the voltage divider. For adjustable parts
Selectable Output (Fixed Voltage Parts)
The Select pin (available only on LT1500-3/5) allows the
user to select either a 3.3V or 5V output. Floating the pin
sets output voltage at 3.3V and grounding the pin sets
output voltage at 5V. The equivalent circuit of the Select
pin function is shown in Figure 2.
R
2
12 1 265
1 1 265
Figure 2. Schematic of Select Pin Function
M
– .
AMPLIFIER
Figure 1. External Voltage Divider
GND
.
ERROR
AMPLIFIER
ERROR
–
+
1.265V
–
+
118
1.265V
k
FB
LT1500/LT1501
V
V
OUT
OUT
204k
69k
58k
= 12V
R1
1M
1%
R2
118K
1%
LTC1500/01 • F01
LTC1500/01 • F02
SELECT
OUT
pin. This
11