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LT1578IS8 Datasheet PDF : 28 Pages
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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Peak-to-peak output ripple voltage is the sum of a triwave
created by peak-to-peak ripple current times ESR, and a
square wave created by parasitic inductance (ESL) and
ripple current slew rate. Capacitive reactance is assumed
to be small compared to ESR or ESL.
( )( ) ( ) VRIPPLE =
IP-P
ESR
+
ESL
Σ dI
dt
Example: with VIN =10V, VOUT = 5V, L = 30µH, ESR = 0.1Ω,
ESL = 10nH:
(5)(10 − 5)
( ) IP-P =
10
30
•
10−6
200
•
103
= 0.42A
Σ dI = 10 = 0.33 • 106
dt 30 • 10−6
( )( ) VRIPPLE = 0.42A 0.1 + 10 • 10−9 0.33 • 106
= 0.042 + 0.003 = 45mVP-P
20mV/DIV
VOUT AT
IOUT = 1A
200mA/DIV
20mV/DIV
INDUCTOR
CURRENT
AT IOUT = 1A
VOUT AT
IOUT = 50mA
200mA/DIV
2µs/DIV
INDUCTOR
CURRENT
AT IOUT = 50mA
1578 F03
Figure 3. LT1578 Ripple Voltage Waveform
CATCH DIODE
The suggested catch diode (D1) is a 1N5818 Schottky, or
its Motorola equivalent, MBR130. It is rated at 1A average
forward current and 30V reverse voltage. Typical forward
voltage is 0.42V at 1A. The diode conducts current only
during switch off time. Peak reverse voltage is equal to
regulator input voltage. Average forward current in normal
operation can be calculated from:
( ) ( ) ID
AVG
= IOUT
VIN − VOUT
VIN
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
VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 1.8A:
( ) 1.8 15 − 4
( ) ID AVG = 15 = 1.32A
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
13
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