LTC1624 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1624' PDF : 28 Pages View PDF
LTC1624
APPLICATIONS INFORMATION
100Ω resistor in series with the SENSE – pin. This offset
cancels the internal offset in current comparator I2 (refer
to Functional Diagram). This comparator in conjunction
with the voltage on the ITH/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 IOUT > IOUT(MIN).
Step-Down Converter: Design Example
As a design example, assume VIN = 12V(nominal),
VIN = 22V(max), VOUT = 3.3V and IMAX = 2A. RSENSE can
immediately be calculated:
RSENSE = 100mV/2A = 0.05Ω
Assume a 10µH inductor. To check the actual value of the
ripple current the following equation is used:
( )( ) ∆IL
=
VIN −
f
VOUT
L
VOUT + VD
VIN + VD
The highest value of the ripple current occurs at the
maximum input voltage:
( ) ∆IL =
22V − 3.3V
200kHz 10µH
3.3V + 0.5V
22V + 0.5V
= 1.58AP-P
The power dissipation on the topside MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in: RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input
voltage with T(estimated) = 50°C:
PMAIN =
( ) [ ( )( )]( ) 3.3V + 0.5V
2
2A 1+
0.005
50°C − 25°C
0.042Ω
22V + 0.5V
( ) ( )( )( ) 1.85
+ 2.5 22V 2A 100pF 200kHz = 62mW
The most stringent requirement for the Schottky diode
occurs when VOUT = 0V (i.e. short circuit) at maximum VIN.
In this case the worst-case dissipation rises to:
( )( ) PD = ISC AVG
VD
VIN
VIN + VD
With the 0.05Ω sense resistor ISC(AVG) = 2A will result,
increasing the 0.5V Schottky diode dissipation to 0.98W.
CIN is chosen for an RMS current rating of at least 1.0A at
temperature. COUT 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:
VORIPPLE = RESR(∆IL) = 0.03Ω (1.58AP-P) = 47mVP-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 VOUT to lose regulation. Avoiding cycle skipping
imposes a limit on the input voltage for a given output
voltage only when VOUT < 2.2V using 30V MOSFETs.
(Remember not to exceed the absolute maximum voltage
of 36V.)
VIN(MAX) = 11.1VOUT + 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.
VINSENSE–
BOOST
LTC1624
TG
SW
GND
VIN
+
RSENSE
CIN
L1
D1
VOUT
M1
CB
R2 +
VFB
COUT
R1
1624 F06
Figure 6. Boost Converter
13
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