LTC3523-2_15 Datasheet, PDF (11/16 Pages) Linear Technology – Synchronous 600mA Step-Up and 400mA Step-Down DC/DC Converters

LTC3523/LTC3523-2
APPLICATIONS INFORMATION
will increase loop bandwidth resulting in a faster transient
response. Maximum capacitance values will produce lower
ripple. Table 3 shows a list of several ceramic capacitor
manufacturers. Consult the manufacturers directly for
detailed information on their entire selection of ceramic
parts.
Table 2. Step-Down Output Capacitor Range vs Programmed
Output Voltage
V_OUT MINIMUM CAPACITANCE (μF) MAXIMUM CAPACITANCE (μF)
0.8
8.4
33.6
1.2
5.6
22.4
1.5
4.5
17.9
1.8
3.7
14.9
2.5
2.7
10.7
5
1.3
5.4
Table 3. Capacitor Vendor Information
SUPPLIER
PHONE
AVX
[phone]
Murata
[phone]
Taiyo-Yuden
[phone]
WEBSITE
www.avxcorp.com
www.murata.com
www.t-yuden.com
STEP-UP V_IN > V_OUT OPERATION
The LTC3523/LTC3523-2 step-up converters will maintain
voltage regulation when the input voltage is above the
output voltage. Since this mode will dissipate more power,
the maximum output current is limited in order to maintain
an acceptable junction temperature and is given by:
( ) ^IOUT(MAX)
=
136
•
⎡⎣
250 – T_A
V_IN + 1.5
–
V_OUT ⎤⎦
T
where T_A = ambient temperature.
For example, at V_IN = 4.5V, V_OUT = 3.3V and T_A = 85°C, the
maximum output current is limited to 449mA.
SHORT-CIRCUIT PROTECTION
The LTC3523/LTC3523-2’s step-up output disconnect
feature allows output short circuit while maintaining
a maximum internally set current limit. However, the
LTC3523/LTC3523-2 also incorporate internal features
such as current limit foldback and thermal shutdown for
protection from an excessive overload or short circuit.
During a prolonged short circuit of V_OUT less than 950mV,
the current limit folds back to 2/3 the normal current limit.
This 2/3 current limit remains in effect until V_OUT exceeds
1V, at which time the normal internal set current limit is
restored.
When the LTC3523/LTC3523-2 step-down converters out-
put is shorted to ground, the step-down uses a comparator
to limit the current through the synchronous rectifying
N-channel switch to 650mA. If this limit is exceeded, the
P-channel switch is inhibited from turning on until the
current through the synchronous rectifying N-channel
switch falls below 650mA.
THERMAL CONSIDERATIONS
To deliver the LTC3523/LTC3523-2’s full-rated power, it is
imperative that a good thermal path be provided to dis-
sipate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal pad
on the underside of the LTC3523/LTC3523-2. It is recom-
mended that multiple vias in the printed circuit board be
used to conduct heat away from the LTC3523/LTC3523-2
and into the copper plane with as much area as possible.
In the event that the junction temperature gets too high,
the LTC3523/LTC3523-2 will go into thermal shutdown
and all switching will cease until the internal temperature
drops to a safe level at which point the soft-start cycle
will be initiated.
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