el7556d Intersil Corporation, el7556d Datasheet - Page 9

• Manufacturer Part Number: el7556d
• Description: Integrated Adjustable 6 Amp Synchronous Switcher
• Manufacturer: Intersil Corporation

Block Diagram
Applications Information
Circuit Description
General
The EL7556D is a fixed frequency, current mode controlled
DC:DC converter with integrated N-channel power
MOSFETS and a high precision reference. The device
incorporates all of the active circuitry required to implement a
cost effective, user-programmable 6A synchronous buck
converter suitable for use in CPU power supplies. By
combining fused-lead packaging technology with an efficient
synchronous switching architecture, high power outputs
(21W) can be realized without the use of discrete external
heat sinks.
Theory of Operation
The EL7556D is composed of 7 major blocks:
1. PWM Controller
2. Output Voltage Mode Select
3. NMOS Power FETS and Drive Circuitry
4. Bandgap Reference
5. Oscillator
6. Temperature Sensor
7. Power Good and Power On Reset
VCCDET, Pin 13
CSLOPE, Pin 3
COSC, Pin 4
OUTEN, Pin 14
CREF, Pin 27
FB2, Pin 28
FB1, Pin 1
VDD
1.26V
9
4V
+
+
-
-
UVLO
S
R
2-1 MUX
+
-
S
EL7556D
Current Limit
VDD
R
SS
+
+
-
-
Current Sense
C
+
+
-
-
SS
PWM
+
-
PWM Controller
The EL7556D regulates output voltage through the use of
current-mode controlled pulse width modulation. The three
main elements in a PWM controller are the feedback loop
and reference, a pulse width modulator whose duty cycle is
controlled by the feedback error signal, and a filter which
averages the logic level modulator output. In a step-down
(buck) converter, the feedback loop forces the time-
averaged output of the modulator to equal the desired output
voltage. Unlike pure voltage-mode control systems current-
mode control utilizes dual feedback loops to provide both
output voltage and inductor current information to the
controller. The voltage loop minimizes DC and transient
errors in the output voltage by adjusting the PWM duty-cycle
in response to changes in line or load conditions. Since the
output voltage is equal to the time-average of the modulator
output the relatively large LC time constants found in power
supply applications generally results in low bandwidth and
poor transient response. By directly monitoring changes in
inductor current via a series sense resistor the controller’s
response time is not entirely limited by the output LC filter
and can react more quickly to changes in line or load
conditions. This feed-forward characteristic also simplifies
AC loop compensation since it adds a zero to the overall
loop response. Through proper selection of the current-
feedback to voltage-feedback ratio, the overall loop
response will approach a one pole system. The resulting
system offers several advantages over traditional voltage
LEB T
R
S
R
DELAY
FF
Q
Q
S
Zero Cross Detect
V2X
Over Temp
Sensor
+
-
PWRGD, Pin 16
CP, Pin 27
C2V, Pin 26
VHI, Pin 24
VDD and VIN,
Pin 5,6,8
LX, Pin 20-23
VSSP, Pin 9-
12, 18-19
OT, Pin 15
V
SS,
Pin 25