LTC1700 Linear Technology, LTC1700 Datasheet - Page 9

• Manufacturer Part Number: LTC1700
• Description: No RSENSE Synchronous Step-Up DC/DC Controller
• Manufacturer: Linear Technology

APPLICATIONS
load current. When the LTC1700 is operating in continu-
ous mode, the duty cycles for the MOSFETs are:
The MOSFET power dissipations at maximum output
current are:
Both MOSFETs have I
includes an additional term for transition losses, which are
largest at high output voltages. The constant k = 2.5 can be
used to estimate the amount of transition loss. The syn-
chronous MOSFET losses are greatest at high input volt-
age and low output voltage.
Start-Up Load Current
In start-up mode, the current limit is set at 60mA and the
oscillator runs at 210kHz with 50% duty cycle at
V
energy that is stored in the inductor during the on time is
small. Therefore the LTC1700 is incapable of supplying
the full load current. Figure 4 shows the amount of load
current the LTC1700 can provide while successfully exit-
ing out of the start-up mode. If the load current exceeds
the amount shown in Figure 4 during start-up, the output
voltage will not increase but will “hang” at a value below
the regulated voltage. However, if the load current is lower,
IN
Main MOSFET Duty Cycle = 1 – V
Synchronous MOSFET Duty Cycle = V
P
+ (k)(V
P
= 1.8V. Since the current limit is low, the amount of
MAIN
SYNC
= (V
= (1 – V
0UT
2
40
35
30
25
20
15
10
IN
5
0
)(I
1.0
Figure 4. Start-Up Load Current
/V
O(MAX)
A = 15 H
B = 10 H
C = 6.2 H
D = 4.2 H
E = 2.2 H
OUT
1.2
IN
U
/V
)(I
2
OUT
)C
1.4
R losses and the P
O(MAX)
INFORMATION
RSS
U
)(I
1.6
V
O(MAX)
(f)
IN
2
(V)
)(
1.8
T(BOT)
2
W
2.0
)(
IN
/V
T(MAIN)
2.2
)(R
OUT
1700 • G04
A
B
C
D
E
IN
MAIN
DS(ON)
/V
2.4
OUT
)(R
U
equation
)
DS(ON)
)
then there is a net positive amount of energy stored in the
output capacitor for every cycle. The output voltage then
rises and once it exceeds 2.3V, the LTC1700 will success-
fully exit out of its start-up mode.
Operating Frequency and Synchronization
The choice of operating frequency and inductor value is a
trade-off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses, both gate charge loss and
transition loss. However, lower frequency operation re-
quires more inductance for a given amount of ripple
current.
The internal oscillator runs at a nominal 530kHz frequency
when the SYNC/MODE pin is either connected to GND or
V
SYNC/MODE pin, the internal oscillator will lock on to the
external clock. The LTC1700 uses a novel technique to
phase lock to the external clock without the requirement of
an external PLL filter, hence minimizing components. The
capture range is between 400kHz to 750kHz. Do not
synchronize below or above the capture range as this will
cause abnormal operation. During synchronization, Burst
Mode operation is inhibited.
The LTC1700 will lock on at the leading edge of the external
clock and the minimum pulse width required is
200ns.
Remember just because you can operate at a high switch-
ing frequency doesn’t always mean you should. At higher
frequencies the switching loss increases, so the C
the N-channel MOSFET becomes very critical to keep
efficiencies high.
Slope Compensation and Peak Inductor Current
Current mode switching regulators that operate with a
duty cycles greater than 50% with continuous inductor
current can exhibit duty cycle instability. While the regu-
lator will not be damaged and may even continue to
function acceptably, a look at its frequency spectrum will
indicate harmonics. These harmonics may interfere with
other sensitive devices and will cause non-optimal perfor-
mance.
IN
. When a CMOS compatible clock is applied to the
LTC1700
RSS
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