LT1952/LT1952-1
20
19521fe
For SD_V
SEC
= 1.32V, fOSC = 200kHz and R
DELAY
= 40k
This gives k = 1 and t
DELAY
= 40ns.
Re-arranging the above equation to solve for SS_MAXDC
= V
SS(REG)
= [0.6 + (t
DELAY
•f
OSC
)(SD_V
SEC
)]/(k•0.522)
=[0.6+(40ns•200kHz)(1.32V)]/(1•0.522)
= (0.608)(1.32)/0.522 = 1.537V
Step 3: Calculate t(V
SS(REG)
) – t(V
SS(ACTIVE)
)
Recall the time for SS_MAXDC to charge to a given voltage
V
SS
is given by:
t = R
CHARGE
•C
SS
•(–1)•ln(1–V
SS
/SS_MAXDC(DC))
(Figure 11 gives the model for SS_MAXDC charging)
For R
T
= 35.7k, R
B
= 100k, R
CHARGE
= 26.3k
For C
SS
= 0.1µF, this gives t(V
SS(ACTIVE)
)
= t(V
SS(0.8V)
) = 2.63e
4
•1e
–7
•(–1)•ln(1–0.8/1.84)
= 2.63e
–3
•(–1)•ln(0.565)=1.5e
–3
s
t(V
SS(REG)
) = t(V
SS(1.537V)
)=26.3k•0.1µF•–1•
ln(1 – 1.66/1.84) = 2.63e
–3
•(–1)•ln(0.146)
= 5e
–3
s
The rise time for the converter output
= t(V
SS(REG)
) – t(V
SS(ACTIVE)
) = (5 – 1.5)e
–3
s
= 3.5e
–3
s
Example (3) Time For Maximum Duty Cycle Clamp to
Reach Within X% of Target Value
A maximum duty cycle clamp of 72% was calculated
previously in the section ‘Programming Maximum
Duty Cycle Clamp’. The programmed value used for
SS_MAXDC(DC) was 1.84V.
The time for SS_MAXDC to charge from its minimum value
V
SS(MIN)
to within X% of SS_MAXDC(DC) is given by:
t(SS_MAXDC charge time within X% of target)
=t[(1–(X/100)•SS_MAXDC(DC)]–t(V
SS(MIN)
)
For X = 2 and V
SS(MIN)
=0.45V,t(0.98•1.84)–
t(0.45) = t(1.803) – t(0.45)
From previous calculations, t(0.45) = 7.3e – 4 s.
Using previous values for R
T
, R
B
, and C
SS
,
APPLICATIONS INFORMATION
t(1.803) = 2.63e
–4
•1e
–7
•(–1)•ln(1–1.803/1.84)
= 2.63e
–3
•(–1)•ln(0.02)=1.03e
–2
s
Hence the time for SS_MAXDC to charge from its minimum
reset threshold of 0.45V to within 2% of its target value
is given by:
t(1.803) – t(0.45) =
1.03e
–2
– 7.3e
–4
= 9.57e
–3
Forward Converter Applications
The following section covers applications where the
LT1952/LT1952-1 are used in conjunction with other LTC
parts to provide highly efficient power converters using
the single switch forward converter topology.
95% Efficient, 5V, Synchronous Forward Converter
The circuit in Figure 14 is based on the LT1952-1 to provide
the simplest forward power converter circuit—using only
one primary MOSFET. The SOUT pin of the LT1952-1
provides a synchronous control signal for the LTC1698
located on the secondary. The LTC1698 drives secondary
side synchronous rectifier MOSFETs to achieve high
efficiency. The LTC1698 also serves as an error amplifier
and optocoupler driver.
Efficiency and transient response are shown in Figures 12
and 13. Peak efficiencies of 95% and ultra-fast transient
response are superior to presently available power
modules. Integrated soft-start, overcurrent detection and
short-circuit hiccup mode provide low stress, reliable
protection. In addition, the circuit in Figure 14 is an all-
ceramic capacitor solution providing low output ripple
voltage and improved reliability. The LT1952-based
converter can be used to replace power module converters
at a much lower cost. The LT1952 solution benefits from
thermal conduction of the system board resulting in higher
efficiencies and lower rise in component temperatures. The
7mm height allows dense packaging and the circuit can
easily be adjusted to provide an output voltage from 1.23V
to 26V. Higher currents are achievable by simple scaling
of power components. The LT1952-1-based solution in
Figure 14 is a powerful topology for replacement of a wide
range of power modules.
