LT8708
27
Rev 0
INTERNAL OSCILLATOR
The operating frequency of the LT8708 can be set using
the internal free-running oscillator. When the SYNC pin
is driven low (< 0.5V), the operating frequency is set by
the value of the resistor from the RT pin to ground. An
internally trimmed timing capacitor resides inside the IC.
The oscillator frequency is calculated using the following
formula:
f
OSC
=
43,750
R
T
+1
⎛
⎝
⎜
⎞
⎠
⎟
kHz
where:
f
OSC
is in kHz and R
T
is in kΩ.
Conversely, R
T
(in kΩ) can be calculated from the desired
frequency (in kHz) using:
R
T
=
43,750
f
OSC
– 1
⎛
⎝
⎜
⎞
⎠
⎟
kΩ
SYNC PIN AND CLOCK SYNCHRONIZATION
The operating frequency of the LT8708 can be synchronized
to an external clock source. To synchronize to the external
source, simply provide a digital clock signal into the SYNC
pin. The LT8708 will operate at the SYNC clock frequency.
The duty cycle of the SYNC signal must be between 20%
and 80% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
1. SYNC may not toggle outside the frequency range of
100kHz to 400kHz unless it is stopped low to enable
the free-running oscillator.
2. The SYNC pin frequency can always be higher than
the free-running oscillator set frequency, f
OSC
, but
should not be less than 25% below f
OSC
.
After SYNC begins toggling, it is recommended that
switching activity is stopped before the SYNC pin stops
toggling. Excess inductor current can result when SYNC
stops toggling as the LT8708 transitions from the external
SYNC clock source to the internal free-running oscillator
clock. Switching activity can be stopped by driving either
the SWEN or
SHDN
pin low.
CLKOUT PIN AND CLOCK SYNCHRONIZATION
The CLKOUT pin can drive up to 200pF and toggles
at the LT8708’s internal clock frequency whether the
internal clock is synchronized to the SYNC pin or is
free-running based on the external R
T
resistor. The rising
edge of CLKOUT is approximately 180° out of phase
from the internal clock’s rising edge or the SYNC pin’s
rising edge if it is toggling. CLKOUT starts toggling
when the INITIALIZE state is entered (see Figure 2).
The CLKOUT pin can be used to synchronize other devices
to the LT8708’s switching frequency. For example, the
CLKOUT pin can be tied to the SYNC pin of another LT8708
regulator which will operate approximately 180°out of
phase of the master LT8708. The frequency of the master
LT8708 can be set by the external R
T
resistor or by toggling
the SYNC pin. Note that the R
T
pin of the slave LT8708
must have a resistor tied to ground. In general, use the
same value R
T
resistor for all of the synchronized LT8708s.
The duty cycle of CLKOUT is proportional to the die tem-
perature and can be used to monitor the die for thermal
issues. See the Junction Temperature Measurement section
for more information.
INDUCTOR CURRENT SENSING AND SLOPE
COMPENSATION
The LT8708 operates using inductor current mode control.
As described previously in the Power Switch Control sec-
tion, the LT8708 measures the peak of the inductor current
waveform in the boost region and the valley of the inductor
current waveform in the buck region. The inductor current
is sensed across the R
SENSE
resistor with pins CSP and
CSN. During any given cycle, the peak (boost region) or
valley (buck region) of the inductor current is controlled
by the V
C
pin voltage.
Slope compensation provides stability in constant-frequency
current mode control architectures by preventing subhar-
monic oscillations at high duty cycles. This is accomplished
internally by adding a compensating ramp to the inductor
current signal in the boost region, or subtracting a ramp
from the inductor current signal in the buck region. At higher
duty cycles, this results in a reduction of maximum inductor
current in the boost region, and an increase of the maximum
APPLICATIONS INFORMATION
