Datasheet

13
LTC1624
100Ω resistor in series with the SENSE
–
pin. This offset
cancels the internal offset in current comparator I
2
(refer
to Functional Diagram). This comparator in conjunction
with the voltage on the I
TH
/RUN pin determines when to
enter into Burst Mode operation (refer to Low Current
Operation in Operation section). With the additional exter-
nal offset present, the drive to the topside MOSFET is
always enabled every cycle and constant frequency opera-
tion occurs for I
OUT
> I
OUT(MIN)
.
Step-Down Converter: Design Example
As a design example, assume V
IN
= 12V(nominal),
V
IN
= 22V(max), V
OUT
= 3.3V and I
MAX
= 2A. R
SENSE
can
immediately be calculated:
R
SENSE
= 100mV/2A = 0.05Ω
Assume a 10µH inductor. To check the actual value of the
ripple current the following equation is used:
∆I
VV
fL
VV
VV
L
IN OUT OUT D
IN D
=
−
()()
+
+






The highest value of the ripple current occurs at the
maximum input voltage:
∆I
VV
kHz H
VV
VV
L
=
−
()
+
+






=
22 3 3
200 10
33 05
22 0 5
158
...
.
.
µ
A
P-P
The power dissipation on the topside MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in: R
DS(ON)
= 0.042Ω, C
RSS
= 100pF. At maximum input
voltage with T(estimated) = 50°C:
P
VV
VV
ACC
V A pF kHz mW
MAIN
=
+
+
()
+
()
°− °
()
[]
()
+
()()( )( )
=
33 05
22 0 5
2 1 0 005 50 25 0 042
2 5 22 2 100 200 62
2
185
..
.
..
.
.
Ω
The most stringent requirement for the Schottky diode
occurs when V
OUT
= 0V (i.e. short circuit) at maximum V
IN
.
In this case the worst-case dissipation rises to:
PI V
V
VV
D SC AVG D
IN
IN D
=
()
+






()
APPLICATIONS INFORMATION
WUU
U
With the 0.05Ω sense resistor I
SC(AVG)
= 2A will result,
increasing the 0.5V Schottky diode dissipation to 0.98W.
C
IN
is chosen for an RMS current rating of at least 1.0A at
temperature. C
OUT
is chosen with an ESR of 0.03Ω for low
output ripple. The output ripple in continuous mode will be
highest at the maximum input voltage. The output voltage
ripple due to ESR is approximately:
V
ORIPPLE
= R
ESR
(∆I
L
) = 0.03Ω (1.58A
P-P
) = 47mV
P-P
Step-Down Converter: Duty Cycle Limitations
At high input to output differential voltages the on-time
gets very small. Due to internal gate delays and response
times of the internal circuitry the minimum recommended
on-time is 450ns. Since the LTC1624’s frequency is inter-
nally set to 200kHz a potential duty cycle limitation exists.
When the duty cycle is less than 9%, cycle skipping may
occur which increases the inductor ripple current but does
not cause V
OUT
to lose regulation. Avoiding cycle skipping
imposes a limit on the input voltage for a given output
voltage only when V
OUT
< 2.2V using 30V MOSFETs.
(Remember not to exceed the absolute maximum voltage
of 36V.)
V
IN(MAX)
= 11.1V
OUT
+ 5V For DC > 9%
Boost Converter Applications
The LTC1624 is also well-suited to boost converter appli-
cations. A boost converter steps up the input voltage to a
higher voltage as shown in Figure 6.
Figure 6. Boost Converter
+
C
B
L1
M1
R2
R1
R
SENSE
C
IN
D1
V
IN
1624 F06
V
IN
V
FB
LTC1624
SENSE
–
BOOST
TG
SW
GND
+
C
OUT
V
OUT
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