LT1578 Linear Technology, LT1578 Datasheet - Page 14

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1578/LT1578-2.5
APPLICATIONS
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 0.33 F 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
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;
less than 0.5V (a slightly arbitrary number) at the worst-
case condition of t
0.24 F. 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:
14
V = (t
ON
)(25mA/C). To keep capacitor ripple voltage to
INPUT
ON
U
C1
= 4.7 s, the capacitor needs to be
INFORMATION
U
R
R
LO
HI
IN
SHDN
W
LT1578
Figure 4. Undervoltage Lockout
3.5 A
OUT
U
/ 50. At
2.42V
0.4V
GND
R
FB
–
+
+
–
f = Switching frequency
V
V
This formula can yield capacitor values substantially less
than 0.24 F, 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 LT1578. Typically, UVLO is used in situations where
the input supply is current limited , or has a relatively high
source resistance. It is particularly useful for input sup-
plies with foldback current limiting. 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 and latch under low source voltage
OUT
IN
C
= Minimum input voltage
MIN
= Regulated output voltage
I
STANDBY
TOTAL
SHUTDOWN
OUT
f V
/
50
OUT
V
SW
V
OUT
3
V
/
V
IN
+
OUTPUT
1578 F04