ISL8009AIRZ-TK Intersil, ISL8009AIRZ-TK Datasheet - Page 10

• Manufacturer Part Number: ISL8009AIRZ-TK
• Description: IC REG SYNC BUCK 1.5A 8-DFN
• Manufacturer: Intersil
• Type: Step-Down (Buck)r

Specifications of ISL8009AIRZ-TK

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5.5 V
• Current - Output: 1.5A
• Frequency - Switching: 1.6MHz
• Voltage - Input: 2.7 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

Theory of Operation
The ISL8009A is a step-down switching regulator
optimized for battery-powered handheld applications.
The regulator operates at 1.6MHz fixed switching
frequency under heavy load condition to allow small
external inductor and capacitors to be used for minimal
printed-circuit board (PCB) area. At light load, the
regulator reduces the switching frequency, unless forced
to the fixed frequency to minimize the switching loss and
to maximize the battery life. The quiescent current when
the output is not loaded is typically only 17µA. The
supply current is typically only 0.1µA when the regulator
is shutdown.
PWM Control Scheme
The ISL8009A employes the current-mode pulse-width
modulation (PWM) control scheme for fast transient
response and pulse-by-pulse current limiting. Figure 25
shows the block diagram. The current loop consists of the
oscillator, the PWM comparator COMP, the current
sensing circuit, and the slope compensation for the
current loop stability. The current sensing circuit consists
of the resistance of the P-Channel MOSFET when it is
turned on and the current sense amplifier CSA. The gain
for the current sensing circuit is typically 0.4V/A. The
control reference for the current loops comes from the
error amplifier EAMP of the voltage loop.
The PWM operation is initialized by the clock from the
oscillator. The P-Channel MOSFET is turned on at the
beginning of a PWM cycle and the current in the
MOSFET starts to ramp up. When the sum of the current
amplifier CSA and the compensation slope (0.675V/µs)
reach the control reference of the current loop, the PWM
comparator COMP sends a signal to the PWM logic to
turn off the P-MOSFET and to turn on the N-Channel
MOSFET. The N-MOSFET stays on until the end of the
PWM cycle. Figure 26 shows the typical operating
waveforms during the PWM operation. The dotted lines
illustrate the sum of the compensation ramp and the
current-sense amplifier CSA output.
CLOCK
Clock
V
OUT
I
0
L
8 CYCLES
8 Cycles
10
Nominal + 1.5%
NOMINAL +1.5%
FIGURE 27. SKIP MODE OPERATION WAVEFORMS
ISL8009A
LOAD CURRENT
CURRENT LIMIT
NOMINAL
Load Current
Current Limit
Nominal
The output voltage is regulated by controlling the
reference voltage to the current loop. The bandgap
circuit outputs a 0.8V reference voltage to the voltage
control loop. The feedback signal comes from the VFB
pin. The soft-start block only affects the operation during
the start-up and will be discussed separately. The error
amplifier is a transconductance amplifier that converts
the voltage error signal to a current output. The voltage
loop is internally compensated with the 30pF and 300kΩ
RC network. The maximum EAMP voltage output is
precisely clamped to the bandgap voltage (1.172V).
SKIP Mode
The ISL8009A enters a pulse-skipping mode at light load
to minimize the switching loss by reducing the switching
frequency. Figure 27 illustrates the skip-mode operation.
A zero-cross sensing circuit shown in Figure 25 monitors
the N-MOSFET current for zero crossing. When 8
consecutive cycles of the N-MOSFET crossing zero are
detected, the regulator enters the skip mode. During the
8 detecting cycles, the current in the inductor is allowed
to become negative. The counter is reset to zero when
the current in any cycle does not cross zero.
CYCLE
Cycle
V
V
DUTY
Duty
V
EAMP
CSA1
OUT
I
L
FIGURE 26. PWM OPERATION WAVEFORMS
November 19, 2009
FN6656.2