ISL78213 Datasheet, PDF (11/15 Pages) Intersil Corporation – 3A Low Quiescent Current, High Efficiency Synchronous Buck Regulator

ISL78213
Typical Operating Performance
Unless otherwise noted, operating conditions are: T_A = +25°C, V_VIN = 2.5V to 5.5V, EN = V_IN, SYNCH = 0V, L = 1.5µH,
C₁ = 2x22µF, C₂ = 2x22µF, I_OUT = 0A to 3A. (Continued)
PHASE 2V/DIV
LX 2V/DIV
V_OUT 0.5V/DIV
IL 2A/DIV
PG 5V/DIV
V_OUT 1V/DIV
IL 2A/DIV
PG 5V/DIV
FIGURE 33. OUTPUT SHORT CIRCUIT
FIGURE 34. OUTPUT SHORT CIRCUIT RECOVERY
5.000
4.875
4.750
4.625
OCP_3.3V_IN
4.500
4.375
4.250
OCP_5V_IN
4.125
4.000
-50
-25
0
25
50
75
100
TEMPERATURE (°C)
FIGURE 35. OUTPUT CURRENT LIMIT vs TEMPERATURE
Theory of Operation
The ISL78213 is a step-down switching regulator
optimized for battery-powered handheld applications.
The regulator operates at 1MHz fixed switching
frequency under heavy load conditions to allow smaller
external inductors 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 35µA. The
supply current is typically only 0.1µA when the regulator
is shut down.
PWM Control Scheme
Pulling the SYNCH pin HI (>2.5V) forces the converter
into PWM mode, regardless of output current. The
ISL78213 employs the current-mode pulse-width
modulation (PWM) control scheme for fast transient
response and pulse-by-pulse current limiting. Figure 2
shows the block diagram. The current loop consists of the
oscillator, the PWM comparator, current sensing circuit
and the slope compensation for the current loop stability.
The gain for the current sensing circuit is typically
250mV/A. The control reference for the current loops
comes from the error amplifier's (EAMP) output.
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 slope compensation (237mV/µs)
reaches 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 turn on the N-Channel
MOSFET. The N-MOSFET stays on until the end of the
PWM cycle. Figure 36 shows the typical operating
waveforms during the PWM operation. The dotted lines
illustrate the sum of the slope compensation ramp and
the current-sense amplifier’s CSA output.
The output voltage is regulated by controlling the V_EAMP
voltage to the current loop. The bandgap circuit outputs
a 0.8V reference voltage to the voltage 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
11
FN7550.1
March 16, 2010