LT1952 Linear Technology, LT1952 Datasheet - Page 15

• Manufacturer Part Number: LT1952
• Description: Single Switch Synchronous Forward Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Programming Synchronous Rectifier Timing:
SOUT to OUT delay (‘t
The LT1952 has an additional output SOUT which pro-
vides a ±50mA peak drive clamped to 12V. In applications
requiring synchronous rectification for high efficiency, the
LT1952 SOUT provides a sync signal for secondary side
control of the synchronous rectifier MOSFETs (Figure11).
Timing delays through the converter can cause non-
optimum control timing for the synchronous rectifier
MOSFETs. The LT1952 provides a programmable delay
(t
rising edge to optimize timing control for the synchronous
rectifier MOSFETs to achieve maximum efficiency gains. A
resistor R
sets the value of t
from 10ns with R
(see graph in Typical Performance Characteristics)
Programming Maximum Duty Cycle Clamp
For forward converter applications using the simplest
topology of a single MOSFET on the primary, a maximum
switch duty cycle clamp which adapts to transformer input
voltage is necessary for reliable control of the MOSFET.
This volt-second clamp provides a safeguard for trans-
former reset that prevents transformer saturation. The
LT1952 SD_V
tor-less, programmable volt-second clamp solution using
simple resistor ratios (Figure 9).
An increase of voltage at the SD_V
maximum duty cycle clamp to decrease. Deriving SD_V
from a resistor divider connected to system input voltage
creates the volt-second clamp. The maximum duty cycle
clamp can be adjusted by programming voltage on the
DELAY
Figure 8. Programming SOUT to OUT Delay: t
, Figure 8) between SOUT rising edge and OUT
SOUT
OUT
DELAY
SEC
t
DELAY
connected from the DELAY pin to ground
DELAY
and SS_MAXDC pins provide a capaci-
DELAY
U
= 10k to 160ns with R
DELAY
. Typical values for t
U
’)
LT1952
DELAY
1952 F08
W
SEC
pin causes the
R
DELAY
DELAY
DELAY
DELAY
U
= 160k.
range
SEC
SS_MAXDC pin using a resistor divider from V
increase of voltage at the SS_MAXDC pin causes the
maximum duty cycle clamp to increase.
To program the volt-second clamp, the following steps
should be taken:
(1) The maximum operational duty cycle of the converter
(2) An initial value for the maximum duty cycle clamp
Note: Since maximum operational duty cycle occurs at
minimum system input voltage (UVLO), the voltage at the
SD_V
where,
(3) The maximum duty cycle clamp calculated in (2)
should be programmed to be 10% greater than the
maximum operational duty cycle calculated in (1). Simple
adjustment of maximum duty cycle can be achieved by
adjusting SS_MAXDC.
Max Duty Cycle Clamp (OUT pin)
= k • 0.522(SS_MAXDC(DC)/SD_V
(t
SS_MAXDC(DC) = V
SD_V
t
k = 1.11 – 5.5e
DELAY
should be calculated for the given application.
should be calculated using the equation below with a
first pass guess for SS_MAXDC.
DELAY
SEC
Figure 9. Programming Maximum Duty Cycle Clamp
SEC
CLAMP INPUT
DUTY CYCLE
pin = 1.32V.
= programmed delay between SOUT and OUT
• f
ADAPTIVE
= 1.32V at minimum system input voltage
OSC
)
*MINIMUM ALLOWABLE R
GUARANTEE SOFT-START PULL-OFF
–7
INPUT VOLTAGE
• (f
SYSTEM
REF
OSC
R1
R2
(R
)
B
R
/(R
R
B
T
*
www.DataSheet4U.com
T
T
MAX DUTY CYCLE
CLAMP ADJUST INPUT
+ R
IS 10k TO
SD_V
SS_MAXDC
V
REF
SEC
B
LT1952
SEC
)
) –
LT1952
1952 F09
15
REF
. An
1952f