LTC3527 Linear Technology Corporation, LTC3527 Datasheet - Page 12

• Manufacturer Part Number: LTC3527
• Description: 1.2MHz/2.2MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: Linear Technology Corporation

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LTC3527/LTC3527-1
OPERATION
the output voltage droops slightly, switching resumes.
This maximizes effi ciency at very light loads by minimiz-
ing switching and quiescent current losses. Burst Mode
output voltage ripple, which is typically 1% peak-to-peak,
can be reduced by using more output capacitance (10µF
or greater), or with a small capacitor (10pF to 50pF) con-
nected between V
If either load current increases, the LTC3527/LTC3527-1
will automatically leave Burst Mode operation. Note that
larger output capacitor values may cause this transition
to occur at lighter loads. Once the LTC3527/LTC3527-1
APPLICATIONS INFORMATION
V
The LTC3527/LTC3527-1 will maintain output voltage regu-
lation even when the input voltage is above one or both of
the desired output voltages. Since this mode is less effi cient
and will dissipate more power in the LTC3527/LTC3527-1,
the maximum output current capability is limited in order
to maintain an acceptable junction temperature. When
operating with V
To maintain a junction temperature below 125°C, the fol-
lowing formula must be adhered to:
where T
Short-Circuit Protection
The LTC3527/LTC3527-1 output disconnect feature allows
an output short-circuit while maintaining a maximum in-
ternally set current limit. The converters also incorporate
internal features such as current limit foldback and thermal
shutdown for protection from an excessive overload or
short circuit. To reduce power dissipation under short-
circuit conditions, the peak switch current limit is reduced
to 500mA (typical) for converter 1 and 350mA (typical) for
converter 2 when V
12
IN
P
(
P
> V
OUT
OUT
OUT
A
1
=
is the ambient temperature.
+
I
Operation
OUT
P
OUT
⎡ ⎣
IN
2
(
OUT1
V
)
> V
OUT
IN
68
OUT
+
(V
°
is less than 1V.
C W
1 5 .
OUT2
/
the power is defi ned by:
)
−
=
) and FB1 (FB2).
V
125
OUT
−
⎤ ⎦
T
A
has left Burst Mode operation and returned to normal
operation, it will remain there until both output loads are
reduced below the burst threshold current.
Burst Mode operation is inhibited during start-up and soft-
start and until both V
greater than V
When the MODE pin is high, LTC3527/LTC3527-1 features
continuous PWM fi xed frequency operation at 1.2MHz
(FSEL = Low) or 2.2MHz (FSEL = High). At very light
loads, the LTC3527/LTC3527-1 will exhibit pulse-skip
operation.
Schottky Diode
Although it is not required, adding a Schottky diode from
SW1 (SW2) to V
about 2%. Note that this defeats the output disconnect
and short-circuit protection features.
PCB Layout Guidelines
The high speed operation of the LTC3527/LTC3527-1
demands careful attention to board layout. A careless
layout will result in reduced performance. Figure 1 shows
the recommended component placement. A large ground
pin copper area will help to lower the die temperature. A
multilayer board with a separate ground plane is ideal, but
not absolutely necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3527/LTC3527-1 can utilize small surface mount
inductors due to their fast 1.2MHz/2.2MHz switching
frequencies. Inductor values between 3.3µH and 4.7µH
are suitable for most 1.2MHz applications. Inductor values
between 1.5µH and 2.2µH are suitable for most 2.2MHz
applications. Larger values of inductance will allow slightly
greater output current capability (and lower the Burst
IN
if neither channel is in shutdown.
OUT1
OUT1
(V
OUT2
and V
) will improve effi ciency by
OUT2
are at least 0.24V
35271f