LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 28

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3810
voltage V
forced until V
Table 1.
MODE/SYNC PIN
DC Voltage: 0V to 0.75V
DC Voltage: ≥ 0.85V
Feedback Resistors
Ext. Clock OV to ≥ 2V
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3810, the maximum sense voltage is controlled
by the voltage on the V
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
The current limit value should be checked to ensure that
I
generally occurs with the largest V
ent temperature, conditions that cause the largest power
loss in the converter. Note that it is important to check for
self-consistency between the assumed MOSFET junction
temperature and the resulting value of I
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
current limit is determined by the minimum MOSFET
on-resistance. Data sheets typically specify nominal
and maximum values for R
A reasonable assumption is that the minimum R
28
LIMIT(MIN)
I
V
LIMIT
OUT2(MIN)
=
OUT2(MIN)
> I
R
V
DS(ON)
OUT(MAX)
SNS(MAX)
OUT2
= 0.8V 1+
DS(ON)
has risen above its minimum.
below which continuous operation is
T
. The minimum value of current limit
RNG
+
CONDITION
Forced Continuous
Current Reversal Enabled
Pulse Skip Mode Operation
No Current Reversal
Regulating a Secondary Winding
Forced Continuous
Current Reversal Enabled
of the MOSFETs. The maximum
2
1
pin. With valley current control,
R4
R3
DS(ON)
I
L
IN
, but not a minimum.
at the highest ambi-
LIMIT
which heats
DS(ON)
lies the same percentage below the typical value as the
maximum lies above it. Consult the MOSFET manufacturer
for further guidelines.
To further limit current in the event of a short-circuit to
ground, the LTC3810 includes foldback current limiting.
If the output falls by more than 60%, then the maximum
sense voltage is progressively lowered to about one tenth
of its full value.
Be aware also that when the fault timeout is enabled for
the external NMOS regulator, an over current limit may
cause the output to fall below the minimum 6.5V UV
threshold. This condition will cause a linear regulator
timeout/restart sequence as described in the Linear Regula-
tor Timeout section if this condition persists.
Soft-Start and Tracking
The LTC3810 has the ability to either soft-start by itself with
a capacitor or track the output of another supply. When
the device is confi gured to soft-start by itself, a capacitor
should be connected to the TRACK/SS pin. The LTC3810 is
put in a low quiescent current shutdown state (I
if the SHDN pin voltage is below 1.5V. The TRACK/SS
pin is actively pulled to ground in this shutdown state.
Once the SHDN pin voltage is above 1.5V, the LTC3810 is
powered up. A soft-start current of 1.4μA then starts to
charge the soft-start capacitor C
is achieved not by limiting the maximum output current
of the controller but by controlling the ramp rate of the
output voltage. Current foldback is disabled during this
soft-start phase. During the soft-start phase, the LTC3810
is ramping the reference voltage until it reaches 0.8V. The
force continuous mode is also disabled and PGOOD signal
is forced low during this phase. The total soft-start time
can be calculated as:
When the device is confi gured to track another supply,
the feedback voltage of the other supply is duplicated
by a resistor divider and applied to the TRACK/SS pin.
Therefore, the voltage ramp rate on this pin is determined
by the ramp rate of the other supply output voltage.
t
SOFTSTART
= 0.8 • C
SS
/1.4μA
SS
. Note that soft-start
Q
~240μA)
3810fb