ISL6269AIRZ-T Datasheet, PDF (10/14 Pages) Intersil Corporation – High-Performance Notebook PWM Controller

ISL6269A
Programming the Output Voltage
When the converter is in regulation there will be 600mV from
the FB pin to the GND pin. Connect a two-resistor voltage
divider across the VO pin and the GND pin with the output
node connected to the FB pin. Scale the voltage-divider
network such that the FB pin is 600mV with respect to the
GND pin when the converter is regulating at the desired
output voltage. The output voltage can be programmed from
600mV to 3.3V.
Programming the output voltage is written as:
V_REF
=
V_O
U
T
•
------------^R----^B----^O-----^T---^T----^O----^M---------------
R_TOP + R_BOTTOM
(EQ. 5)
Where:
- V_OUT is the desired output voltage of the converter
- V_REF is the voltage that the converter regulates to
between the FB pin and the GND pin
- R_TOP is the voltage-programming resistor that connects
from the FB pin to the VO pin. In addition to setting the
output voltage, this resistor is part of the loop
compensation network
- R_BOTTOM is the voltage-programming resistor that
connects from the FB pin to the GND pin
Beginning with R_TOP between 1kΩ to 5kΩ, calculating
R_BOTTOM is written as:
R_BOTTOM
=
--^V----^R----^E----^F----^•---^R----^T----^O-----^P---
V_OUT – V_REF
(EQ. 6)
Programming the PWM Switching Frequency
The ISL6269A 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 ISL6269A 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.
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
ISL6269A
Q_{HIGH_SIDE}
L_OUT
DCR
Q_{LOW_SIDE}
C_OUT
C_ESR
FIGURE 6. COMPENSATION REFERENCE CIRCUIT
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 ISL6269A and the external components
R1, R2, C1, and C2 as well as the frequency setting
components R_FSET, and C_FSET, are identified in the
schematic Figure 6.
General Application Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a single-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced below. In
10
FN9253.2
May 30, 2007