LT1108_15 Datasheet, PDF (8/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1108
APPLICATI S I FOR ATIO
In this mode the switch is arranged in common collector or
step-down mode. The switch drop can be modeled as a
0.75V source in series with a 0.65Ω resistor. When the
switch closes, current in the inductor builds according to
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing V_IN –
V_CESAT to appear across L1. A current then builds up in L1.
At the end of the switch-ON time the current in L1 is
( ) ^I_L
t
=
V_L
R'

^{1 –}
e
–R't
L


(15)
where: R' = 0.65Ω + DCR_L
V_L = V_IN – 0.75V
As an example, suppose – 5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
P_L = (– 5V+ 0.5V)(100mA) = 550mW.
(16)
Energy required from the inductor is
I_PEAK
=
V_{I N}
L
t _ON*
L1
V_IN
R3
I_LIM
V _IN
SW1
LT1108 FB
GND
SW2
(20)
D1
V_OUT
R2
+
C1
R1
P_{L =} 550mW _{= 28.9µJ}
f_OSC 19kHz
(17)
Figure 1. Step-Up Mode Hookup
LT1108 • F01
Picking an inductor value of 220µH with 0.3Ω DCR results
in a peak switch current of
( ) ^{4.5V – 0.75V}  ^{–0.95Ω × 36µs}  ⁽¹⁸⁾
( ) ^{IPEAK = 0.65Ω + 0.3Ω} ^{1– e 220µH} 
= 568mA
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V_OUT + V_D, the
inductor current flows through D1 into C1, increasing V_OUT.
This action is repeated as needed by the LT1108 to keep V_FB
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
Substituting I_PEAK into Equation (04) results in
( ) ( ) ^E_L
=
1
2
220µH
0.568A ² = 35.5µJ
(19)
Since 35.5µJ > 28.9µJ, the 220µH inductor will work.
Finally, R_LIM should be selected by looking at the Switch
Current vs R_LIM curve. In this example, R_LIM = 150Ω.
STEP-UP (BOOST MODE) OPERATION
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
( ) ^V_OUT
=
_{1 +}
R2
R1
1.245V
(21)
STEP-DOWN (BUCK MODE) OPERATION
A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1108 based
step-down converter is shown in Figure 2.
When the switch turns on, SW2 pulls up to V_IN – V_SW. This
puts a voltage across L1 equal to V_IN – V_SW – V_OUT, causing
a current to build up in L1. At the end of the switch- ON time,
the current in L1 is equal to
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
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