LT1934-1 Linear Technology, LT1934-1 Datasheet - Page 8

• Manufacturer Part Number: LT1934-1
• Description: Micropower Step-Down Switching Regulators in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1934/LT1934-1
the average inductor current equals the load current, the
maximum load current is:
where I
to-peak ripple current in the inductor. The ripple current is
determined by the off time, t
value:
I
delay in the control circuitry that results in a higher peak
current and a more accurate value is:
These expressions are combined to give the maximum
load current that the LT1934 will deliver:
The minimum current limit is used here to be conserva-
tive. The third term is generally larger than the second
term, so that increasing the inductor value results in a
higher output current. This equation can be used to evalu-
ate a chosen inductor or it can be used to choose L for a
given maximum load current. The simple, single equa-
tion rule given above for choosing L was found by setting
ing the delay term). Note that this analysis assumes that
the inductor current is continuous, which is true if the
ripple current is less than the peak current or I
8
PK
I
L
I
I
I
• (V
I
• (V
OUT(MAX)
PK
OUT(MAX)
OUT(MAX)
is nominally equal to I
= I
I
L
= I
OUT
OUT
LIM
= (V
PK
LIM
/2.5. This results in I
is the peak inductor current and I
OUT
+ V
+ V
= I
+ 150ns • (V
= 350mA + 150ns • (V
= 90mA + 150ns • (V
D
D
+ V
PK
)/2L (LT1934)
)/2L (LT1934-1)
Table 2. Inductor Vendors
Vendor
Murata
Sumida
Coilcraft
Wurth
Electronics
– I
D
U
) • t
L
OFF
/2
IN
U
LIM
/L
– V
OFF
Phone
[phone]
[phone]
[phone]
[phone]
. However, there is a slight
OUT(MAX)
OUT
= 1.8 s, and the inductor
)/L
W
IN
IN
– V
– V
~0.8I
OUT
OUT
URL
www.murata.com
www.sumida.com
www.coilcraft.com
www.we-online.com
L
)/L – 1.8 s
is the peak-
)/L – 1.8 s
LIM
U
L
(ignor-
< I
PK
.
The inductor must carry the peak current without saturat-
ing excessively. When an inductor carries too much
current, its core material can no longer generate addi-
tional magnetic flux (it saturates) and the inductance
drops, sometimes very rapidly with increasing current.
This condition allows the inductor current to increase at
a very high rate, leading to high ripple current and
decreased overload protection.
Inductor vendors provide current ratings for power induc-
tors. These are based on either the saturation current or on
the RMS current that the inductor can carry without dissi-
pating too much power. In some cases it is not clear which
of these two determine the current rating. Some data
sheets are more thorough and show two current ratings,
one for saturation and one for dissipation. For LT1934
applications, the RMS current rating should be higher than
the load current, while the saturation current should be
higher than the peak inductor current calculated above.
Input Capacitor
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT1934 and to force this switching current into a
tight local loop, minimizing EMI. The input capacitor must
have low impedance at the switching frequency to do this
effectively. A 2.2 F ceramic capacitor (1 F for the
LT1934-1) satisfies these requirements.
If the input source impedance is high, a larger value
capacitor may be required to keep input ripple low. In this
case, an electrolytic of 10 F or more in parallel with a 1 F
ceramic is a good combination. Be aware that the input
Part Series
LQH3C
CR43
CDRH4D28
CDRH5D28
DO1607C
DO1608C
DT1608C
WE-PD1, 2, 3, 4
Comments
Small, Low Cost, 2mm Height
www.DataSheet4U.com
1934f