OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

SC4501MLTRT

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21
10
© 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4501
Application Information
The absolute maximum operating frequency of the
converter is therefore
MHz67.1
ns
150
25
.
0
ns
150
D
MIN
==
. The
actual operating frequency needs to be lower to allow for
modulating headroom.
The power transistor in the SC4501 is turned off every
switching period for an interval determined by the
discharge time of the oscillator ramp and the propagation
delay of the power switch. This minimum off time limits
the maximum duty cycle of the regulator at a given
switching frequency. A boost converter with high
In
OUT
V
V
ratio
requires long switch on time and high duty cycle. If the
required duty cycle is higher than the attainable maximum,
then the converter will operate in dropout. (Dropout is a
condition in which the regulator cannot attain its set
output voltage below current limit.)
The minimum off times of closed-loop boost converters set
to various output voltages were measured by lowering their
input voltages until dropout occurs. It was found that the
minimum off time of the SC4501 ranged from 80 to 110ns
at room temperature.
Beware of dropout when operating at very low input voltages
(1.5-2V) and with off times approaching 110ns. Shorten
the PCB trace between the power source and the device
input pin, as line drop may be a significant percentage of
the input voltage. A regulator in dropout may appear as if
it is in current limit. The cycle-by-cycle current limit of the
SC4501 is duty-cycle and input voltage invariant and is
typically 2.8A. If the switch current limit is not at least 2A,
then the converter is likely in dropout. The switching
frequency should then be lowered to improve controllability.
Both the minimum on time and the minimum off time
reduce control range of the PWM regulator. Bench
measurement showed that reduced modulating range
started to be a problem at frequencies over 2MHz. Although
the oscillator is capable of running well above 2MHz,
controllability limits the maximum operating frequency.
Inductor Selection
The inductor ripple current ΔI
L
of a boost converter
operating in continuous-conduction mode is
()
fL
V
V
D
I
CESATIN
L
−
=Δ
(5)
where f is the switching frequency and L is the inductance.
Substituting (3) into (5) and neglecting V
CESAT
,
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−=Δ
DOUT
ININ
L
VV
V
1
fL
V
I
(6)
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough to
lessen jittery tendency but not so steep that large flux swing
decreases efficiency. Inductor ripple current ΔI
L
between
25-40% of the peak inductor current limit is a good
compromise. Inductors so chosen are optimized in size
and DCR. Setting ΔI
L
= 0.3•(2) = 0.6A, V
D
=0.5V in (6),
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−
Δ
=
5.0V
V
1
f6.0
V
VV
V
1
If
V
L
OUT
ININ
DOUT
IN
L
IN
(7)
where L is in μH and f is in MHz.
Equation (6) shows that for a given V
OUT
, ΔI
L
is the highest
when
()
2
V
V
V
DOUT
IN
+
=
. If V
IN
varies over a wide range, then
choose L based on the nominal input voltage.
The saturation current of the inductor should be 20-30%
higher than the peak current limit (2.8A). Low-cost powder
iron cores are not suitable for high-frequency switching
power supplies due to their high core losses. Inductors
with ferrite cores should be used.
Input Capacitor
The input current in a boost converter is the inductor
current, which is continuous with low RMS current ripples.
A 2.2-4.7µF ceramic input capacitor is adequate for most
applications.
Output Capacitor
Both ceramic and low ESR tantalum capacitors can be
used as output filtering capacitors. Multi-layer ceramic
capacitors, due to their extremely low ESR (<5mΩ), are
the best choice. Use ceramic capacitors with stable
temperature and voltage characteristics. One may be
tempted to use Z5U and Y5V ceramic capacitors for
output filtering because of their high capacitance and
2011
11
© 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4501
Application Information
small sizes. However these types of capacitors have high
temperature and high voltage coefficients. For example,
the capacitance of a Z5U capacitor can drop below 60%
of its room temperature value at –25
°C and 90°C. X5R
ceramic capacitors, which have stable temperature and
voltage coefficients, are the preferred type.
The diode current waveform in Figure 5 is discontinuous
with high ripple-content. In a buck converter the inductor
ripple current ΔI
L
determines the output ripple voltage.
The output ripple voltage of a boost regulator is however
much higher and is determined by the absolute inductor
current. Decreasing the inductor ripple current does not
appreciably reduce the output ripple voltage. The current
flowing in the output filter capacitor is the difference
between the diode current and the output current. This
capacitor current has a RMS value of:
1
V
V
I
IN
OUT
OUT
−
(8)
If a tantalum capacitor is used, then its ripple current rating
in addition to its ESR will need to be considered.
When the switch is turned on, the output capacitor supplies
the load current I
OUT
(Figure 5). The output ripple voltage
due to charging and discharging of the output capacitor is
therefore:
OUT
OUT
OUT
C
DT
I
V =Δ
(9)
For most applications, a 10-22µF ceramic capacitor is
sufficient for output filtering. It is worth noting that the
output ripple voltage due to discharging of a 10µF ceramic
capacitor (9) is higher than that due to its ESR.
Rectifying Diode
For high efficiency, Schottky barrier diodes should be used
as rectifying diodes for the SC4501. These diodes should
have a RMS current rating of at least 1A and a reverse
blocking voltage of at least a few Volts higher than the
output voltage. For switching regulators operating at low
duty cycles (i.e. low output voltage to input voltage
conversion ratios), it is beneficial to use rectifying diodes
with somewhat higher RMS current ratings (thus lower
forward voltages). This is because the diode conduction
interval is much longer than that of the transistor.
Converter efficiency will be improved if the voltage drop
across the diode is lower.
The rectifying diodes should be placed close to the SW
pins of the SC4501 to minimize ringing due to trace
inductance. Surface-mount equivalents of 1N5817,
1N5818, MBRM120 (ON Semi) and 10BQ015 (IRF) are
all suitable.
Soft-Start
Soft-start prevents a DC-DC converter from drawing
excessive current (equal to the switch current limit) from
the power source during start up. If the soft-start time is
made sufficiently long, then the output will enter regulation
without overshoot. An external capacitor from the SS pin
to the ground and an internal 1.5µA charging current
source set the soft-start time. The soft-start voltage ramp
at the SS pin clamps the error amplifier output. During
regulator start-up, COMP voltage follows the SS voltage.
The converter starts to switch when its COMP voltage
exceeds 0.7V. The peak inductor current is gradually
increased until the converter output comes into regulation.
If the shutdown pin is forced below 1.1V or if fault is
detected, then the soft-start capacitor will be discharged
to ground immediately.
The SS pin can be left open if soft-start is not required.
Shutdown
The input voltage and shutdown pin voltage must be greater
than 1.4V and 1.1V respectively to enable the SC4501.
Forcing the shutdown pin below 1.1V stops switching.
Pulling this pin below 0.1V completely shuts off the SC4501.
The total V
IN
current decreases to 10µA at 2V. Figure 6
shows several ways of interfacing the control logic to the
shutdown pin. Beware that the shutdown pin is a high
impedance pin. It should always be driven from a low-
impedance source or tied to a resistive divider. Floating
the shutdown pin will result in undefined voltage. In Figure
6(c) the shutdown pin is driven from a logic gate whose
V
OH
is higher than the supply voltage of the SC4501. The
diode clamps the maximum shutdown pin voltage to one
diode voltage above the input power supply.
2011
12
© 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4501
Application Information
Programming Undervoltage Lockout
The SC4501 has an internal V
IN
undervoltage lockout
(UVLO) threshold of 1.4V. The transition from idle to
switching is abrupt but there is no hysteresis. If the input
voltage ramp rate is slow and the input bypass is limited,
then sudden turn on of the power transistor will cause a
dip in the line voltage. Switching will stop if V
IN
falls below
the internal UVLO threshold. The resulting output voltage
rise may be non-monotonic. The 1.1V disable threshold of
the SC4501 can be used in conjunction with a resistive
voltage divider to raise the UVLO threshold and to add an
UVLO hysteresis. Figure 7 shows the scheme. Both V
H
and
V
L
(the desired upper and the lower UVLO threshold
voltages) are determined by the 1.1V threshold crossings,
V
H
and V
L
are therefore:
()
3HYSHHYSHL
4
3
H
RIVVVV
V1.1
R
R
1V
−=−=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
(10)
Re-arranging,
HYS
HYS
3
I
V
R =
(11)
1
1
.
1
V
R
H
3
4
−
=
(12)
Figure 6. Methods of Driving the Shutdown Pin
(c)
1N4148
SC4501
SHDN
IN
V
IN
(a)
SC4501
SHDN
IN
(d)
SC4501
SHDN
IN
(b)
SC4501
SHDN
IN
(a) Directly Driven from a Logic Gate
(b) Driven from an Open-drain N-channel MOSFET or an Open-Collector NPN Transistor (V
OL
< 0.1V)
(c) Driven from a Logic Gate with V
OH
> V
IN
(d) Combining Shutdown with Programmed UVLO (See Section Below).
2011
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21

SC4501MLTRT

Mfr. #:
Buy SC4501MLTRT
Manufacturer:
Semtech
Description:
Switching Voltage Regulators 2AMP,2MHZ STEP-UP SW REG W/SS
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet