LT1952
19
1952f
APPLICATIO S I FOR ATIO
WUU
U
0.6 = k • 0.522(SS_MAXDC(DC)/SD_V
SEC
) –
(t
DELAY
• f
OSC
)
For SD_V
SEC
= 1.32V, f
OSC
= 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
,
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 mini-
mum 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
is 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 to provide
the simplest forward power converter circuit — using only
one primary MOSFET. The SOUT pin of the LT1952 pro-
vides a synchronous control signal for the LTC1698 lo-
cated on the secondary. The LTC1698 drives secondary
side synchronous rectifier MOSFETs to achieve high effi-
ciency. 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 mod-
ules. Integrated soft-start, over-current 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 con-
verter 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 tempera-
tures. 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-based
solution in Figure 14 is a powerful topology for replace-
ment of a wide range of power modules.
