LTC3459_07 Datasheet, PDF (7/12 Pages) Linear Technology – 10V Micropower Synchronous Boost Converter

LTC3459
OPERATION
Operation
The LTC3459 synchronous boost converter utilizes a
Burst Mode control technique to achieve high efficiency
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 profile
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).
( ) ^I_PEAK ^{= I}_LIMIT ⁺
100ns
V_IN
L
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.
V_OUT
AC
RIPPLE
I_PEAK
t_OFF t_OFF t_OFF
~50mV_P-P
~100mA _tOFF
N PNP NPN
BURST ON
P
I_ZERO WAIT
SLEEP
N PN
BURST ON
3459 F01
Figure 1. Inductor Current and V_OUT Ripple Waveforms
110
100
15μH
90
22μH
80
33μH
70
60
50
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V_IN (V)
3459 F02
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5
V_OUT – V_IN (V)
3459 F03
Figure 2. Typical I_PEAK Values
Figure 3. t_OFF Times
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