ISL6268 Datasheet, PDF (10/14 Pages) Intersil Corporation – High-Performance Notebook PWM Controller

ISL6268
Programming the PWM Switching Frequency
The ISL6268 does not use a clock signal to produce PWM.
The PWM switching frequency F_SW is programmed by the
resistor R_FSET that is connected from the FSET pin to the
GND pin. The approximate PWM switching frequency is
written as:
F_SW = _K------_â‹---_R----¹-_F---_S----_E----_T--
(EQ. 7)
Estimating the value of R_FSET is written as:
R_FSET
=
---------¹----------
K • F_SW
(EQ. 8)
where:
- F_SW is the PWM switching frequency
- R_FSET is the F_SW programming resistor
- K = 75 x 10^-12
It is recommended that whenever the control loop
compensation network is modified, F_SW should be checked
for the correct frequency and if necessary, adjust R_FSET.
Compensation Design
The LC output filter has a double pole at its resonant frequency
that causes the phase to abruptly roll downward. The R³
modulator used in the ISL6268 makes the LC output filter
resemble a first order system in which the closed loop stability
can be achieved with a Type II compensation network.
Your local Intersil representative can provide a PC-based
tool that can be used to calculate compensation network
component values and help simulate the loop frequency
response. The compensation network consists of the internal
error amplifier of the ISL6268 and the external components R1,
R2, C1, and C2, as well as the frequency setting components
R_FSET and C_FSET (all identified in Figure 5).
R2
C1
C2
COMP
- ^FB
EA
+
REF
FSET
R³
MODULATOR
VO
VIN
R1
R_FSET
C_FSET
VOUT
VIN
UG
PHASE
GATE DRIVERS
LG
GND
Q_{HIGH_SIDE}
L_OUT
DCR
Q_{LOW_SIDE}
C_OUT
C_ESR
ISL6268
FIGURE 5. COMPENSATION REFERENCE CIRCUIT
10
FN6348.0
August 22, 2006