ISL6224_06 Datasheet, PDF (6/13 Pages) Intersil Corporation – Single Output Mobile-Friendly PWM Controller

ISL6224
grounded. Table 1. shows the configuration for different
modes of operation. Figure 1 below shows plots of the ramp
speed compensation.
300kHz CLOCK
600kHz CLOCK
V_-----i_--n_{-- -}t_--
2T
Vo/4
V_-----i_--n_{-- -}t_--
4T
V_-----i_--n_{-- -}t_--
8T
Vo/8
FIGURE 1. RAMP SPEED COMPENSATION Vo = 2.5V
TABLE 1. CONFIGURATION FOR MODES OF OPERATION
OPERATION PIN 1 CONNECTION PIN 1 POTENTIAL
One-stage 300kHz
Vin
V1 > 4V
Two-stage 300kHz
150K-GND
1V < V1 < 2V
Two-stage 600kHz
GND
V1 < 0.5V
The synchronous converter light-load efficiency is enhanced
by a hysteretic mode of operation which is automatically
engaged at light loads when the inductor current becomes
discontinuous. As the filter inductor resumes continuous
current, the PWM mode of operation is automatically
restored.
The ISL6224 control IC employs an average current mode
control scheme with input voltage feedforward ramp
programming for better rejection of input voltage variations.
Current Sensing and Current Limit Protection
The PWM converter uses the lower MOSFET on-state
resistance, r_DS(ON), as the current-sensing element. This
technique eliminates the need for a current sense resistor
and the associated power losses. If more accurate current
protection is desired, current sense resistors may be used in
series with the lower MOSFET’s source.
A current proportional signal is used to provide average
current mode control and overcurrent protection. The gain in
the current sense circuit is set by the resistor connected from
ISEN (pin 12) to the PHASE node of the buck converter. The
value of this resistor can be estimated by the following
expression:
Risen = -^I--^o---^m------^a---₇-^x--₅--^•--_µ--^R--_A---^d----^s----^o---ⁿ-- – 100
of this resistor can be obtained from the following
expression:
Rocset = ---¹---¹-----^•----^R-----ⁱ--^s---^e----ⁿ----
Ioc • Rdson
where Ioc is the value of overcurrent. The resulting current
out of the ISEN pin through R_ISEN, is used for current
feedback and current limit protection. This is compared with
an internal current limit threshold. When a sampled value of
the output current is determined to be above the current limit
threshold, the PWM drive is terminated and a counter is
initiated. This limits the inductor current build-up and
essentially switches the converter into current-limit mode. If
an overcurrent is detected between 26ms to 53ms later, an
overcurrent shutdown is initiated. If during the 26ms to 53ms
period, an overcurrent is not detected, the counter is reset
and sampling continues as normal.
This current limit scheme has proven to be very robust in
applications like portable computers where fast inductor
current build-up is common due to a large difference
between input and output voltages and a low value of the
inductor.
Light-Load (Hysteretic) Operation
In the light-load (hysteretic) mode the output voltage is
regulated by the hysteretic comparator which regulates the
output voltage by maintaining the output voltage ripple as
shown in Figure 2. In hysteretic mode, the inductor current
flows only when the output voltage reaches the lower limit of
the hysteretic comparator and turns off at the upper limit.
Hysteretic mode saves converter energy at light loads by
supplying energy only at the time when the output voltage
requires it. This mode conserves energy by reducing the
power dissipation associated with continuous switching.
During the time between inductor current pulses, both the
upper and lower MOSFETs are turned off. This is referred to
as ‘diode emulation mode’ because the lower MOSFET
performs the function of a diode. This diode emulation mode
prevents the output capacitor from discharging through the
lower MOSFET when the upper MOSFET is not conducting.
NOTE: the PWM only operation can intentionally be forced by tying
pin 16, FCCM, to VCC.
where Iomax is the maximum inductor current. The value of
R_ISEN should be specified for the expected maximum
operating temperature.
An overcurrent protection threshold is set by an external
resistor connected from OCSET (pin 4) to ground. The value
6
FN9042.8
June 8, 2006