LT3825EFE#TRPBF Linear Technology, LT3825EFE#TRPBF Datasheet - Page 21

• Manufacturer Part Number: LT3825EFE#TRPBF
• Description: IC CNTRLR SYNC 16-TSSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT3825EFE#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 12 ~ 18 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Power - Output: 60W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -

APPLICATIONS INFORMATION
next cycle. This action naturally holds the V
during the current comparator sense action (current mode
switching).
AN19 provides a method for empirically tweaking frequency
compensation. Basically it involves introducing a load
current step and monitoring the response.
Slope Compensation
This part incorporates current slope compensation. Slope
compensation is required to ensure current loop stability
when the DC is greater than 50%. In some switcher con-
trollers, slope compensation reduces the maximum peak
current at higher duty cycles. The LT3825 eliminates this
problem by having circuitry that compensates for the slope
compensation so that maximum current sense voltage is
constant across all duty cycles.
Minimum Load Considerations
At light loads, the LT3825 derived regulator goes into
forced continuous conduction mode. The primary-side
switch always turns on for a short time as set by the
t
load requires, power will flow back into the primary during
the “off” period when the synchronization switch is on.
This does not produce any inherently adverse problems,
though light load efficiency is reduced.
Maximum Load Considerations
The current mode control uses the V
amplified sense resistor voltage as inputs to the current
comparator. When the amplified sense voltage exceeds the
V
In normal use, the peak switch current increases while
FB is below the internal reference. This continues until
V
MOSFET will turn off at the rated 98mV V
repeats on the next cycle.
ON(MIN)
C
C
node voltage, the primary-side switch is turned off.
reaches its 2.56V clamp. At clamp, the primary-side
resistor. If this produces more power than the
C
node voltage and
SENSE
C
voltage stable
level. This
It is possible for the peak primary switch currents as
referred across R
because of the minimum switch-on time blanking. If the
voltage on V
turn-on time, the SFST capacitor is discharged, causing
the discharge of the V
peak current on the next cycle and will reduce overall
stress in the primary switch.
Short-Circuit Conditions
Loss of current limit is possible under certain conditions
such as an output short circuit. If the duty cycle exhib-
ited by the minimum on time is greater than the ratio of
secondary winding voltage (referred-to-primary) divided
by input voltage, then peak current is not controlled at
the nominal value. It ratchets up cycle-by-cycle to some
higher level. Expressed mathematically, the requirement
to maintain short-circuit control is:
where:
Trouble is typically encountered only in applications with a
relatively high product of input voltage times secondary-
to-primary turns ratio and/or a relatively long minimum
switch on time. Additionally, several real world effects such
as transformer leakage inductance, AC winding losses, and
output switch voltage drop combine to make this simple
theoretical calculation a conservative estimate. Prudent
design evaluates the switcher for short-circuit protection
and adds any additional circuitry to prevent destruction.
t
I
NSP = secondary-to-primary turns ratio (N
Other variables as previously defined
DC
ON(MIN)
SC
MIN
= short-circuit output current
=
= primary-side switch minimum on-time
t
ON MIN
SENSE
(
SENSE
exceeds 206mV after the minimum
)
•
C
f
OSC
to exceed the max 98mV rating
capacitor. This then reduces the
<
I
SC
•
(
R
V
SEC
IN
•
+
N
LT3825
S S P
R
DS ON
SEC
(
21
/N
)
)
PRI
3825fc
)