LT8697
applicaTions inForMaTion
Using RCBL as an Output Current Monitor
ꢂable ꢀ. SW ꢁrequency vs Rꢂ Value
f
(MHz)
0.3
0.4
0.5
0.6
0.7
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
R (kΩ)
SW
T
The primary function of the RCBL pin is to set the cable
drop compensation as discussed in the cable drop com-
pensationsectionearlier.However,theRCBLpinproduces
an output voltage that is proportional to the output load
current. The RCBL pin can therefore be used as an output
load monitor. The voltage on the RCBL pin obeys the fol-
lowing relation to USB load current:
140
102
80.6
66.5
56.2
47.5
37.4
30.1
25.5
21.5
18.7
16.5
14.7
VCBL = ILOAD •RSENSE • 20.55
V
CBL
is valid when the LT8697 is switching.
Since the RCBL pin current is part of the cable drop com-
pensationcontrolloop,excessivecapacitiveloadingonthe
RCBL pin can cause USB output voltage overshoot during
load steps. Keep the capacitive loading on the RCBL pin
below 100pF or isolate the load capacitance with 100kΩ
in series between the RCBL pin and the input it is driving,
as shown in Figure 7.
Operating ꢁrequency Selection and ꢂrade-Offs
Selectionoftheoperatingfrequencyisatrade-offbetween
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller
inductor and capacitor values may be used. The disad-
vantages are lower efficiency and a reduced input voltage
range with constant frequency operation.
RCBL
100k
ADC
R
CBL
The highest switching frequency (f
)) for a given
SW(MAX
8697 F07
application can be calculated as follows:
ꢁigure 7. Using the RCBL Pin as Output Current Monitor
5V + VSW(BOT)
fSW(MAX)
=
Setting the Switching ꢁrequency
tON(MIN) • V − VSW(TOP) + VSW(BOT)
IN
The LT8697 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom300kHzto2.2MHz
by using a resistor tied from the RT pin to ground. A table
where V is the typical input voltage, V
and
IN
SW(TOP)
V
are the internal switch drops (~0.3V and ~0.15V
SW(BOT)
showing the necessary R value for a desired switching
respectively, at maximum load) and t
is the mini-
T
ON(MIN)
frequency is in Table 3. The R resistor required for a
mumtopswitchon-time(seetheElectricalCharacteristics
section). This equation shows that a slower switching
T
desired switching frequency can be calculated using the
following equation:
frequency is necessary to accommodate a high V /V
IN OUT
ratio.
43
fSW
RT
=
− 5.2
For transient operation, V may go as high as the abso-
lute maximum rating of 42V regardless of the R value.
IN
T
However, the LT8697 will reduce switching frequency as
necessary to maintain control of inductor current to as-
sure safe operation.
where R is in kΩ and f is the desired switching fre-
T
SW
quency in MHz.
8697p
18
For more information www.linear.com/LT8697
LINEAR_DIMENSIONS [ Linear Dimensions ]
