ISL97649BIRZ-T Просмотр технического описания (PDF) - Renesas Electronics
Номер в каталоге
Компоненты Описание
Список матч
ISL97649BIRZ-T
Renesas Electronics
ISL97649BIRZ-T Datasheet PDF : 20 Pages
| |||
ISL97649B
Applications Information
Enable Control
With VIN > UVLO, all functions in ISL97649B are shut down when
the Enable pin is pulling down. When the voltage at the Enable
pin reaches H threshold, the whole ISL97649B is on.
Frequency Selection
The ISL97649B switching frequency can be user selected to
operate at either a constant 600kHz or 1.2MHz. Lower switching
frequency can save power dissipation when the boost load is very
low and the device is operating in deep discontinuous mode.
Higher switching frequency can allow the use of smaller external
components like inductors and output capacitors. Higher
switching frequency will get higher efficiency within some
loading ranges, depending on VIN, VOUT, and external
components, as shown in Figure 1. Connecting the FREQ pin to
GND sets the PWM switching frequency to 600kHz. Connecting
the FREQ pin to VIN sets the PWM switching frequency to
1.2MHz.
Soft-Start
Soft-start is provided by an internal current source to charge the
external soft-start capacitor. The ISL97649B ramps up the
current limit from 0A to full value as voltage at the SS pin ramps
from 0 to 0.8V. Hence, the soft-start time is 3.2ms when the soft-
start capacitor is 22nF and is 6.8ms for 47nF and 14.5ms for
100nF.
Operation
The boost converter is a current mode PWM converter operating
at either 600kHz or 1.2MHz. It can operate in both discontinuous
conduction mode (DCM) at light load and in continuous
conduction mode (CCM). In continuous conduction current mode,
current flows continuously in the inductor during the entire
switching cycle in steady-state operation. The voltage conversion
ratio in continuous current mode is given by Equation 1:
V-----BV----oI--N-o---s---t = 1-----–-1----D---
(EQ. 1)
where D is the duty cycle of the switching MOSFET.
The boost regulator uses a summing amplifier architecture
consisting of gm stages for voltage feedback, current feedback,
and slope compensation. A comparator looks at the peak
inductor current, cycle by cycle, and terminates the PWM cycle if
the current limit is reached.
An external resistor divider is required to divide the output
voltage down to the nominal reference voltage. Current drawn by
the resistor network should be limited to maintain the overall
converter efficiency. The maximum value of the resistor network
is limited by the feedback input bias current and the potential for
noise being coupled into the feedback pin. A resistor network on
the order of 60k is recommended. The boost converter output
voltage is determined by Equation 2:
VBoost = R-----1--R---+--2--R-----2- VFB
(EQ. 2)
The current through the MOSFET is limited to 1.5APEAK. This
restricts the maximum output current (average) based on
Equation 3:
IOMAX
=
ILMT
–
---2--I--L-
V-V----I-O-N--
(EQ. 3)
where IL is peak-to-peak inductor ripple current, which is set by
Equation 4:
IL = -V---L-I--N-- -fD-s-
(EQ. 4)
where fS is the switching frequency (600kHz or 1.2MHz).
Capacitor
An input capacitor is used to suppress the voltage ripple injected
into the boost converter. A ceramic capacitor with capacitance
larger than 10µF is recommended. The voltage rating of the
input capacitor should be larger than the maximum input
voltage. Table 1 shows some recommended input capacitors.
TABLE 1. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATIONS
CAPACITOR
SIZE MFG
PART NUMBER
10µF/6.3V
0603 TDK
C1608X5R0J106M
10µF/16V
1206 TDK
C3216X7R1C106M
10µF/10V
0805 Murata
GRM21BR61A106K
22µF/10V
1210 Murata
GRB32ER61A226K
Inductor
The boost inductor is a critical part that influences the output
voltage ripple, transient response, and efficiency. Values of
3.3µH to 10µH are used to match the internal slope
compensation. The inductor must be able to handle the average
and peak currents shown in Equation 5:
ILAVG = 1----I-–-O----D---
ILPK = ILAVG + ---2--I--L-
(EQ. 5)
Table 2 shows some recommended inductors for different design
considerations.
TABLE 2. BOOST INDUCTOR RECOMMENDATIONS
DIMENSIONS
INDUCTOR
(mm)
MFG
PART
DESIGN
NUMBER CONSIDERATION
10µH/
4Apeak
8.3x8.3x4.5 Sumida CDR8D43- Efficiency
100NC optimization
6.8µH/ 5.0x5.0x2.0 TDK PLF5020T-
1.8Apeak
6R8M1R8
10µH/ 6.6x7.3x1.2 Cyntec PCME061B- PCB space /profile
2.2Apeak
100MS optimization
Rectifier Diode
A high-speed diode is necessary due to the high switching
frequency. Schottky diodes are recommended because of their
fast recovery time and low forward voltage. The reverse voltage
rating of this diode should be higher than the maximum output
FN7927 Rev 2.00
June 27, 2013
Page 10 of 20
Share Link: