FAN5234_10 Datasheet, PDF (8/15 Pages) Fairchild Semiconductor – Dual Mobile-Friendly PWM / PFM Controller

V_CORE
I_L
0
V_CORE
I_{L 0}
PWMMode
HystereticMode
12345678
HystereticMode
1
2
3
4
5
6
7
Figure 4. Transitioning between PWM and Hysteretic Mode
PWMMode
8
The hysteretic comparator causes HDRV turn-on when
the output voltage (at VSEN) falls below the lower
threshold (10mV below VREF) and terminates the PFM
signal when VSEN rises over the higher threshold (5mV
above VREF).
The switching frequency is primarily a function of:
ƒ Spread between the two hysteretic thresholds
ƒ
ILOAD
ƒ Output inductor and capacitor ESR
A transition back to PWM Continuous Conduction Mode
or (CCM) occurs when the inductor current rises
sufficiently to stay positive for eight consecutive cycles.
This occurs when:
I
= ⎜⎛ ^{ΔVHYSTERESIS} ⎟⎞
⎝ ^{LOAD( CCM )}
2 ESR
⎠
(3)
where ΔVHYSTERESIS = 15mV and ESR is the equivalent
series resistance of COUT.
Due to different control mechanisms, the value of the
load current where transition into PWM operation takes
place is typically higher compared to the load level at
which transition into Hysteretic Mode occurs. Hysteretic
Mode can be disabled by setting the FPWM pin HIGH.
Current Processing
The following discussion refers to Figure 6.
The current through RSENSE resistor (ISNS) is sampled
shortly after Q2 is turned on. That current is held and
summed with the output of the error amplifier. This
effectively creates a current-mode control loop. The
resistor connected to the ISNS pin (RSENSE) sets the
gain in the current feedback loop. Equation 4 estimates
the recommended value of RSENSE as a function of the
maximum load current (ILOAD(MAX)) and the value of the
MOSFET RDS(ON). RSENSE must be kept higher than
700Ω even if the number calculated comes out less
than 700Ω:
R_SENSE
=
⎜⎜⎝⎛
I_LOAD(MAX) × R_DS(ON)
150μA
− 100 _{⎟⎟⎠⎞}
(4)
Setting the Current Limit
A ratio of ISNS is also compared to the current
established when a 0.9V internal reference drives the
ILIM pin:
R _LIM
=
11
I_LOAD
x_{⎜⎜⎜⎝⎛}
(100 + R_SENSE
R_{DS(ON )}
)
⎟⎞
⎟⎟⎠
(5)
Since the tolerance on the current limit is largely
dependent on the ratio of the external resistors, it is
fairly accurate if the voltage drop on the switching node
side of RSENSE is an accurate representation of the load
current. When using the MOSFET as the sensing
element, the variation of RDS(ON) causes proportional
variation in ISNS. This value varies from device to device
and has a typical junction temperature coefficient of
about 0.4%/°C (consult the MOSFET datasheet for
actual values), the actual current limit set point
decreases proportional to increasing MOSFET die
temperature. A factor of 1.6 in the current limit set point
should compensate for all MOSFET RDS(ON) variations,
assuming the MOSFET’s heat sinking keep its
operating die temperature below 125°C.
Q2
LDRV
ISNS ^RSENSE
PGND
Figure 5. Improving Current-Sensing Accuracy
© 2004 Fairchild Semiconductor Corporation
FAN5234 • Rev. 2.0.0
8
www.fairchildsemi.com