SC4501STDEVB Semtech, SC4501STDEVB Datasheet - Page 9

• Manufacturer Part Number: SC4501STDEVB
• Description: EVALUATION BOARD
• Manufacturer: Semtech

Specifications of SC4501STDEVB

• Silicon Manufacturer: Semtech
• Application Sub Type: Step Up Switching Regulator
• Kit Application Type: Power Management - Voltage Regulator
• Silicon Core Number: SC4501
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Duty Cycle
The duty cycle D of a boost converter is:
where V
voltage drop across the rectifying diode.
Maximum Output Current
In a boost switching regulator the inductor is connected
to the input. The DC inductor current is the input current.
When the power switch is turned on, the inductor current
flows into the switch. When the power switch is off, the
inductor current flows through the rectifying diode to the
output. The output current is the average diode current.
The diode current waveform is trapezoidal with pulse width
(1 – D)T (Figure 5). The output current available from a
boost converter therefore depends on the converter
operating duty cycle. The power switch current in the
SC4501 is internally limited to 2A. This is also the maximum
inductor or the input current. By estimating the conduction
losses in both the switch and the diode, an expression of
the maximum available output current of a boost converter
can be derived:
where I
Application Information
POWER MANAGEMENT
D
2005 Semtech Corp.
I
OUTMAX
Figure 5. Current Waveforms in a Boost Regulator
1
1
LIM
Inductor Current
CESAT
is the switch current limit.
V
V
OUT
OUT
I
V
LIM
V
CESAT
is the switch saturation voltage and V
ON
V
OUT
IN
V
IN
V
V
D
D
1
OFF
ON
DT
45
D
(1-D)T
OFF
ON
V
D
D
OFF
ON
V
V
D
IN
V
Switch Current
ON
CESAT
Diode Current
I IN
I OUT
D
(3)
(4)
is
9
It is worth noting that I
ratio
output current at high frequencies (>1MHz) since
switching losses are neglected in its derivation.
Nevertheless it is a useful first-order approximation.
Using V
the maximum output currents for three V
combinations are shown in Table 1.
Considerations for High Frequency Operation
The operating duty cycle of a boost converter decreases as
V
current-mode switching regulator is the sensed current ramp
of the control switch. This current ramp is absent unless
the switch is turned on. The intersection of this ramp with
the output of the voltage feedback error amplifier
determines the switch pulse width. The propagation delay
time required to immediately turn off the switch after it
is turned on is the minimum switch on time. Regulator
closed-loop measurement shows that the SC4501 has
a minimum on time of about 150ns at room temperature.
The power switch in the SC4501 is either not turned on
at all or for at least 150ns. If the required switch on time
is shorter than the minimum on time, the regulator will
either skip cycles or it will start to jitter.
Example: Determine the maximum operating frequency
of a Li-ion cell to 5V converter using the SC4501.
Assuming that V
the minimum duty ratio can be found using (3).
Table 1. Calculated Maximum Output Current [ Equation (4)]
IN
D
approaches V
MIN
V
V
V
OUT
CESAT
IN
IN
2.5
3.3
5
( V )
1
1
. Equation (4) over-estimates the maximum
= 0.3V, V
5
5
4
0
2 .
3 .
0
0
D
V
=0.5V, V
5 .
5 .
OUT
OUT
12
12
. The PWM modulating ramp in a
D
5
( V )
= 0.5V and I
OUTMAX
. 0
25
CESAT
is directly proportional to the
=0.3V and V
0.820
0.423
0.615
D
LIM
= 2A in (3) and (4),
I
OUTMAX
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0.35
0.76
1.14
IN
SC4501
=2.6 - 4.2V,
IN
( A )
and V
OUT