LTC1625
APPLICATIONS INFORMATION
U
W U U
7V
V
V
OUT
TopDutyCycle =
IN
V – V
IN
OUT
BottomDutyCycle =
1.2V
V
IN
1µs
4µs
The MOSFET power dissipations at maximum output
current are:
±
1625 F04
0
Figure 4. SYNC Clock Waveform
2
V
V
OUT
P
=
(I
)(ρ )(R
T(TOP) DS(ON)
)
TOP
O(MAX)
IN
Inductor Value Selection
2
+ (k)(V )(I
)(C )(f)
IN O(MAX) RSS
Given the desired input and output voltages, the inductor
value and operating frequency directly determine the
ripple current:
2
V – V
IN
OUT
P
=
(I
)(ρ
)(R
)
BOT
O(MAX)
T(BOT)
DS(ON)
V
IN
V
V
OUT
V
IN
OUT
(f)(L)
∆I =
1–
L
Both MOSFETs have I2R losses and the PTOP equation
includesanadditionaltermfortransitionlosses, whichare
largest at high input voltages. The constant k = 1.7 can be
usedtoestimatetheamountoftransitionloss. Thebottom
MOSFETlossesaregreatestathighinputvoltageorduring
a short circuit when the duty cycle is nearly 100%.
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with small ripple current. To achieve this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of IO(MAX). Note that the largest ripple
current occurs at the highest VIN. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tor should be chosen according to:
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
requires more inductance for a given amount of ripple
current.
V
V
OUT
OUT
L ≥
1–
(f)(∆I
)
V
IN(MAX)
L(MAX)
Theinternaloscillatorrunsatanominal150kHzfrequency
when the SYNC pin is left open or connected to ground.
Pulling the SYNC pin above 1.2V will increase the fre-
quency by 50%. The oscillator will injection lock to a clock
signal applied to the SYNC pin with a frequency between
165kHz and 200kHz. The clock high level must exceed
1.2V for at least 1µs and no longer than 4µs as shown in
Figure 4. The top MOSFET turn-on will synchronize with
the rising edge of the clock.
Burst Mode Operation Considerations
The choice of RDS(ON) and inductor value also determines
the load current at which the LTC1625 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
30mV
I
=
BURST(PEAK)
R
DS(ON)
10
Linear [ Linear ]
