LTC3459
7
3459fc
OPERATION
Operation
The LTC3459 synchronous boost converter utilizes a
Burst Mode control technique to achieve high effi ciency
over a wide dynamic range. A 2.5% accurate comparator
is used to monitor the output voltage (V
OUT
), if V
OUT
is
above the comparator threshold, no switching occurs and
only quiescent current (10μA) is drawn from the power
source. When V
OUT
drops below the comparator threshold,
switching commences and the output capacitor is charged.
During the on time of the switching period, inductor cur-
rent is ramped through an internal N-channel MOSFET to
GND until a peak current (75mA) is detected. A P-channel
MOSFET connects the inductor to V
OUT
during the off time
delivering energy to the load. The off time is controlled by
an internal timer which is proportional to 1/(V
OUT
– V
IN
).
Anticross conduction circuitry ensures the N- and P-chan-
nel switches are never on simultaneously.
Only three power components and two feedback resistors
are required to complete the design of the boost converter,
an external Schottky diode is not required. The high op-
erating frequency allows the use of low value, low profi le
inductors and tiny external ceramic capacitors. The boost
converter disconnects V
OUT
from V
IN
during shutdown to
avoid loading the input power source.
Peak Current Overshoot
The LTC3459’s peak current comparator has a delay of ap-
proximately 100ns from the time inductor current reaches
current limit until the internal N-channel MOSFET turns off.
This delay causes the peak current to overshoot based on
the inductor value and V
IN
, as follows (Figure 2 is based
on a 65mA initial I
LIMIT
).
II ns
V
L
PEAK LIMIT
IN
=+
()
100
t
OFF
Timer
The LTC3459’s t
OFF
timer is designed to keep the inductor
current continuous during a Burst Mode switching packet,
thereby increasing current capability at the output. A larger
inductor value will have lower peak-to-peak current ripple,
increasing the available current to the load. This improve-
ment is offset somewhat by the reduced I
PEAK
overshoot.
The t
OFF
timer is designed to maintain a relatively constant
peak-to-peak current in the inductor despite V
IN
changes.
Figure 1. Inductor Current and V
OUT
Ripple Waveforms
Figure 2. Typical I
PEAK
Values Figure 3. t
OFF
Times
V
OUT
AC
RIPPLE
I
PEAK
3459 F01
~50mV
P-P
~100mA
SLEEPI
ZERO
WAITBURST ON
t
OFF
P
t
OFF
PP
t
OFF
P
t
OFF
PNNNN NN
BURST ON
V
IN
(V)
1.5
60
50
I
PEAK
(mA)
70
80
90
100
2.5 3.5
4.5
5.5
3459 F02
110
23
4
5
15μH
22μH
33μH
V
OUT
– V
IN
(V)
0.5
0
t
OFF
(μs)
0.1
0.3
0.4
0.5
0.7
2.5
4.5
5.5
3459 F03
0.2
0.8
0.6
1.5 3.5
6.5
7.5
8.5
