Datasheet
LTC1922-1
14
3. Slope compensation is required for most peak current
mode controllers in order to prevent subharmonic oscilla-
tion of the current control loop. In general, if the system
duty cycle exceeds 50% in a fixed frequency, continuous
current mode converter, an unstable condition exists
within the current control loop. Any perturbation in the
current signal is amplified by the PWM modulator result-
ing in an unstable condition. Some common manifesta-
tions of this include alternate pulse nonuniformity and
pulse width jitter. Fortunately, this can be addressed by
adding a corrective slope to the current sense signal or by
subtracting the same slope from the current command
signal (error amplifier output). In theory, the current
doubler output configuration does not require slope com-
pensation since the output inductor duty cycles only
approach 50%. However, transient conditions can mo-
mentarily cause higher duty cycles and therefore, the
possibility for unstable operation. The exact amount of
required slope compensation is easily programmed by the
LTC1922-1 with the addition of a single external resistor
(see Figure 7). The LTC1922-1 generates a current that is
proportional to the instantaneous voltage on C
T
,
OPERATIO
U
(33µA/V
(CT)
). Thus, at the peak of C
T
, this current is
approximately 125µA and is output from the CS pin. A
resistor connected between CS and the external current
sense resistor sums in the required amount of slope
compensation. The value of this resistor is dependent on
several factors including minimum V
IN
, V
OUT
, switching
frequency, current sense resistor value and output induc-
tor value. An illustrative example with the design equation
is provided below.
Example: V
IN
= 36V to 72V
V
OUT
= 3.3V
I
OUT
= 40A
L = 2.2µH
Transformer turns ratio (N) = V
IN(MIN)
• D
MAX
/
V
OUT
␣=␣3
R
CS
= 0.025Ω
f
SW
= 300kHz, i.e., transformer f = f
SW
/2 = 150kHz
R
SLOPE
= V
O
• R
CS
/(2 • L • f
T
•
125µA • N) = 3.3V • 0.025/
(2 • 2.2µA • 100k • 125µA • 3)
R
SLOPE
= 500Ω, choose the next higher standard value
to account for tolerances in I
SLOPE
, R
CS
, N and L.
LTC1922-1
LTC1922-1
LTC1922-1
C
T
C
T
C
T
C
T
C
T
C
T
SYNC
SYNC
SYNC
5.1k
5.1k
5.1k
1k
1k
•
•
•
UP TO
5 SLAVES
SLAVES
MASTER
C
T
OF SLAVE(S) IS
1.25 C
T
OF MASTER.
1922 F06a
LTC1922-1
C
T
C
T
SYNC
5.1k
1k
1922 F06b
EXTERNAL
FREQUENCY
SOURCE
AMPLITUDE > 1.8V
12.5ns < PW < 0.4/ƒ
Figure 6a. SYNC Output (Master Mode)
Figure 6b. SYNC Input from an External Source
Figure 7. Slope Compensation Circuitry
2. The LTC1922-1 can either synchronize other PWMs, or
be synchronized to an external frequency source or PWM
chip. See Figure 6 for details.
Current Sensing and Overcurrent Protection
Current sensing provides feedback for the current mode
control loop and protection from overload conditions. The
LTC1922-1 is compatible with either resistive sensing or
current transformer methods. Internally connected to the
LTC1922-1 CS pin are two comparators that provide
pulse-by-pulse and overcurrent shutdown functions re-
spectively. (See Figure 8)
BRIDGE
CURRENT
CURRENT SENSE
WAVEFORM
V(C
T
)
33k
I =
CS
C
T
33k
ADDED
SLOPE
R
SLOPE
R
CS
1922 F07
LTC1922-1