LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 15

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the
output capacitor. In addition, V
currents are reduced to typically 25µA and 150µA, respec-
tively, during the sleep time. As the load current decreases
towards a no load condition, the percentage of time that
the LT3437 operates in sleep mode increases and the
average input current is greatly reduced, resulting in
higher efficiency.
The minimum average input current depends on the V
V
network and Schottky diode leakage. It can be approxi-
mated by the following equation:
where
Example: For V
During the sleep portion of the Burst Mode cycle, the V
pin voltage is held just below the level needed for normal
operation to improve transient response. See the Typical
Performance Characteristics section for burst and tran-
sient response waveforms.
OUT
I
V
V
I
I
I
η = low current efficiency (non Burst Mode operation)
I
I
IN AVG
VINS
BIASS
FB
S
IN AVG
OUT
IN
(
= catch diode reverse leakage at V
(
ratio, V
= feedback network current
= input voltage
= input pin current in sleep mode
= output voltage
= BIAS pin current in sleep mode
)
)
=
≅
(
≅
150
C
25
I
25
VINS
frequency compensation, feedback divider
OUT
µ + µ +
µ + µ +
µ +
A
A
A
+
= 3.3V, V
U
I
SHDN
5
5
12 5
(
0 75
A
A
.
.
U
+
µ +
)
60
A
⎛
⎝ ⎜
⎛
⎜
⎝
IN
3 3
V
12
µ =
= 12V
V
OUT
.
0. .
A
IN
IN
⎞
⎠ ⎟
5
W
⎞
⎟
⎠
µ
and BIAS quiescent
90
A
(
I
BIASS
)
µ
OUT
A
( )
+
η
U
I
FB
+
I
S
IN
)
to
C
If Burst Mode operation is undesirable, it can be defeated
by placing 2V or greater on the SYNC pin. When Burst
Mode operation is defeated, output ripple at light loads will
be reduced at the expense of light load efficiency.
CATCH DIODE
The catch diode carries load current during the SW off
time. The average diode current is therefore dependent on
the switch duty cycle. At high input to output voltage
ratios, the diode conducts most of the time. As the ratio
approaches unity, the diode conducts only a small fraction
of the time. The most stressful condition for the diode is
when the output is short circuited. Under this condition,
the diode must safely handle I
To maximize high and low load current efficiency, a fast
switching diode with low forward drop and low reverse
leakage should be used. Low reverse leakage is critical to
maximize low current efficiency since its value over tem-
perature can potentially exceed the magnitude of the
LT3437 supply current. Low forward drop is critical for
high current efficiency since the loss is proportional to
forward drop.
These requirements result in the use of a Schottky type
diode. DC switching losses are minimized due to its low
forward voltage drop, and AC behavior is benign due to its
lack of a significant reverse recovery time. Schottky diodes
are generally available with reverse voltage ratings of 60V,
and even 100V, and are price competitive with other types.
200
180
160
140
120
100
80
60
40
20
0
0
10
20
Figure 5. I
INPUT VOLTAGE (V)
30
40
PEAK
Q
vs V
50
at maximum duty cycle.
IN
V
60
OUT
= 3.3V
70
3435 F05
80
LT3437
15
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