14
LT1576/LT1576-5
APPLICATIONS INFORMATION
WUU
U
This formula will not yield values higher than 1A with
maximum load current of 1.25A unless the ratio of input to
output voltage exceeds 5:1. The only reason to consider a
larger diode is the worst-case condition of a high input
voltage and
overloaded
(not shorted) output. Under short-
circuit conditions, foldback current limit will reduce diode
current to less than 1A, but if the output is overloaded and
does not fall to less than 1/3 of nominal output voltage,
foldback will not take effect. With the overloaded condi-
tion, output current will increase to a typical value of 1.8A,
determined by peak switch current limit of 2A. With
V
IN
= 15V, V
OUT
= 4V (5V overloaded) and I
OUT
= 1.8A:
IA
D AVG
()
=
−
()
=
1 8 15 4
15
132
.
.
This is safe for short periods of time, but it would be
prudent to check with the diode manufacturer if continu-
ous operation under these conditions must be tolerated.
BOOST␣ PIN␣ CONSIDERATIONS
For most applications, the boost components are a 0.33µF
capacitor and a 1N914 or 1N4148 diode. The anode is
connected to the regulated output voltage and this gener-
ates a voltage across the boost capacitor nearly identical
to the regulated output. In certain applications, the anode
may instead be connected to the unregulated input volt-
age. This could be necessary if the regulated output
voltage is very low (< 3V) or if the input voltage is less than
6V. Efficiency is not affected by the capacitor value, but the
capacitor should have an ESR of less than 1Ω to ensure
that it can be recharged fully under the worst-case condi-
tion of minimum input voltage. Almost any type of film or
ceramic capacitor will work fine.
WARNING!
Peak voltage on the BOOST pin is the sum of
unregulated input voltage plus the voltage across the
boost capacitor. This normally means that peak BOOST
pin voltage is equal to input voltage plus output voltage,
but
when the boost diode is connected to the regulator
input, peak BOOST pin voltage is equal to twice the input
voltage. Be sure that BOOST pin voltage does not exceed
its maximum rating.
For nearly all applications, a 0.33µF boost capacitor works
just fine, but for the curious, more details are provided
here. The size of the boost capacitor is determined by
switch drive current requirements. During switch on time,
drain current on the capacitor is approximately I
OUT
/ 50. At
peak load current of 1.25A, this gives a total drain of 25mA.
Capacitor ripple voltage is equal to the product of on time
and drain current divided by capacitor value;
∆V = (t
ON
)(25mA/C). To keep capacitor ripple voltage to
less than 0.5V (a slightly arbitrary number) at the worst-
case condition of t
ON
= 4.7µs, the capacitor needs to be
0.24µF. Boost capacitor ripple voltage is not a critical
parameter, but if the minimum voltage across the capaci-
tor drops to less than 3V, the power switch may not
saturate fully and efficiency will drop. An
approximate
formula for absolute minimum capacitor value is:
C
IVV
fV V
MIN
OUT OUT IN
OUT
=
()( )
()
−
()
//50
3
f = Switching frequency
V
OUT
= Regulated output voltage
V
IN
= Minimum input voltage
This formula can yield capacitor values substantially less
than 0.24µF, but it should be used with caution since it
does not take into account secondary factors such as
capacitor series resistance, capacitance shift with tem-
perature and output overload.
SHUTDOWN FUNCTION AND
UNDERVOLTAGE LOCKOUT
Figure 4 shows how to add undervoltage lockout (UVLO)
to the LT1576. Typically, UVLO is used in situations where
the input supply is
current limited
, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
